<!DOCTYPE html>
<html class="ocks-org do-not-copy" lang="en">

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1" />
  <title>Comprehensive Python Cheatsheet</title>
  <meta name="description" content="Exhaustive, simple, beautiful and concise. A truly Pythonic cheat sheet about Python programming language.">
  <link rel="icon" href="web/favicon.png">

  <link rel="stylesheet" href="web/default.min.css">
  <link rel="stylesheet" href="https://netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.css">
  <link rel="stylesheet" href="web/style.css">

  <script>
    // Uses dark theme 3 if it's specified in query string orbrowser prefers dark mode and 
    // theme is not explicitly set.
    if ((window.location.search.search(/[?&]theme=dark3/) !== -1) || ((window.location.search.search(/[?&]theme=/) == -1) && (window.matchMedia('(prefers-color-scheme: dark)').matches))) {
      document.write("<link rel=\"stylesheet\" href=\"web/default_dark3.min.css\">");
      document.write("<link rel=\"stylesheet\" href=\"web/style_dark3.css\">");
    }
    else if (window.location.search.search(/[?&]theme=dark2/) !== -1) {
      document.write("<link rel=\"stylesheet\" href=\"web/default_dark2.min.css\">");
      document.write("<link rel=\"stylesheet\" href=\"web/style_dark2.css\">");
    }
    else if (window.location.search.search(/[?&]theme=dark1/) !== -1) {
      document.write("<link rel=\"stylesheet\" href=\"web/default_dark1.min.css\">");
      document.write("<link rel=\"stylesheet\" href=\"web/style_dark1.css\">");
    }    
    else if (window.location.search.search(/[?&]theme=dark/) !== -1) {
      document.write("<link rel=\"stylesheet\" href=\"web/default_dark.min.css\">");
      document.write("<link rel=\"stylesheet\" href=\"web/style_dark.css\">");
    }
  </script>

  <meta name="twitter:card" content="summary_large_image">
  <meta name="twitter:title" content="Comprehensive Python Cheatsheet">
  <meta name="twitter:description" content="Exhaustive, simple, beautiful and concise. A truly Pythonic cheat sheet about Python programming language.">
  <meta name="twitter:image" content="https://gto76.github.io/python-cheatsheet/web/image_social_4.png">

  <meta property="og:url" content="https://gto76.github.io/python-cheatsheet/">
  <meta property="og:title" content="Comprehensive Python Cheatsheet">
  <meta property="og:description" content="Exhaustive, simple, beautiful and concise. A truly Pythonic cheat sheet about Python programming language.">
  <meta property="og:site_name" content="gto76.github.io">
  <meta property="og:image" content="https://gto76.github.io/python-cheatsheet/web/image_social_4.png">
  <meta property="og:type" content="article">

  <meta itemprop="url" content="https://gto76.github.io/python-cheatsheet/">
  <meta itemprop="name" content="Comprehensive Python Cheatsheet">
  <meta itemprop="description" content="Exhaustive, simple, beautiful and concise. A truly Pythonic cheat sheet about Python programming language.">
  <meta itemprop="image" content="https://gto76.github.io/python-cheatsheet/web/image_social_4.png">

  <meta name="google-site-verification" content="w3rvuG0D1kUm_w20qsJecSEZh59Am8jK4eSPVU83e_M">
  <meta name="viewport" id="viewport-meta">
</head>

<body>
  <header>
    <aside>November 20, 2025</aside>
    <a href="https://gto76.github.io" rel="author">Jure Šorn</a>
  </header>

   <div><h1 id="comprehensivepythoncheatsheet">Comprehensive Python Cheatsheet</h1><p class="banner"><sup><a href="https://raw.githubusercontent.com/gto76/python-cheatsheet/main/README.md">Download text file</a>, <a href="https://github.com/gto76/python-cheatsheet">Fork me on GitHub</a>, <a href="https://github.com/gto76/python-cheatsheet/wiki/Frequently-Asked-Questions">Check out FAQ</a> or <a href="index.html?theme=dark3">Switch to dark theme</a>.
</sup></p><p class="banner" style="margin-bottom: 20px; padding-bottom: 7px;"><img src="web/image_888.jpeg" alt="Monty Python"></p><script>
  // Changes the banner image and link-to-theme if "theme=dark" is in query string
  // or if browser prefers dark mode and theme is not explicitly set.

const theme_not_set_in_query = window.location.search.search(/[?&]theme=light/) == -1
const browser_prefers_dark = window.matchMedia('(prefers-color-scheme: dark)').matches;

  if ((window.location.search.search(/[?&]theme=dark/) !== -1) || 
      (theme_not_set_in_query && browser_prefers_dark)) {
    activateDarkMode();
  }

  function activateDarkMode() {
    var link_to_theme = document.createElement("a")
    link_to_theme.href = "index.html?theme=light"
    link_to_theme.text = "Switch to light theme"
    document.getElementsByClassName("banner")[0].firstChild.children[3].replaceWith(link_to_theme)

    var img_dark = document.createElement("img");
    img_dark.src = "web/image_orig_blue6.png";
    img_dark.alt = "Monthy Python";
    if ((window.location.search.search(/[?&]theme=dark2/) !== -1) ||
        (window.location.search.search(/[?&]theme=dark3/) !== -1) ||
        (theme_not_set_in_query && browser_prefers_dark)) {
      img_dark.style = "width: 910px;";
    } else {
      img_dark.style = "width: 960px;";
    }
    document.getElementsByClassName("banner")[1].firstChild.replaceWith(img_dark);
  }
</script><pre style="border-left: none;padding-left: 1.9px;"><code class="hljs bash" style="line-height: 1.327em;"><strong>ToC</strong> = {
    <strong><span class="hljs-string"><span class="hljs-string">'1. Collections'</span></span></strong>: [<a href="#list">List</a>, <a href="#dictionary">Dictionary</a>, <a href="#set">Set</a>, <a href="#tuple">Tuple</a>, <a href="#range">Range</a>, <a href="#enumerate">Enumerate</a>, <a href="#iterator">Iterator</a>, <a href="#generator">Generator</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'2. Types'</span></span></strong>:       [<a href="#type">Type</a>, <a href="#string">String</a>, <a href="#regex">Regular_Exp</a>, <a href="#format">Format</a>, <a href="#numbers">Numbers</a>, <a href="#combinatorics">Combinatorics</a>, <a href="#datetime">Datetime</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'3. Syntax'</span></span></strong>:      [<a href="#function">Function</a>, <a href="#inline">Inline</a>, <a href="#imports">Import</a>, <a href="#decorator">Decorator</a>, <a href="#class">Class</a>, <a href="#ducktypes">Duck_Type</a>, <a href="#enum">Enum</a>, <a href="#exceptions">Except</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'4. System'</span></span></strong>:      [<a href="#exit">Exit</a>, <a href="#print">Print</a>, <a href="#input">Input</a>, <a href="#commandlinearguments">Command_Line_Arguments</a>, <a href="#open">Open</a>, <a href="#paths">Path</a>, <a href="#oscommands">OS_Commands</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'5. Data'</span></span></strong>:        [<a href="#json">JSON</a>, <a href="#pickle">Pickle</a>, <a href="#csv">CSV</a>, <a href="#sqlite">SQLite</a>, <a href="#bytes">Bytes</a>, <a href="#struct">Struct</a>, <a href="#array">Array</a>, <a href="#memoryview">Memory_View</a>, <a href="#deque">Deque</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'6. Advanced'</span></span></strong>:    [<a href="#operator">Operator</a>, <a href="#matchstatement">Match_Stmt</a>, <a href="#logging">Logging</a>, <a href="#introspection">Introspection</a>, <a href="#threading">Threading</a>, <a href="#coroutines">Coroutines</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'7. Libraries'</span></span></strong>:   [<a href="#progressbar">Progress_Bar</a>, <a href="#plot">Plot</a>, <a href="#table">Table</a>, <a href="#consoleapp">Console_App</a>, <a href="#guiapp">GUI</a>, <a href="#scraping">Scraping</a>, <a href="#webapp">Web</a>, <a href="#profiling">Profile</a>],
    <strong><span class="hljs-string"><span class="hljs-string">'8. Multimedia'</span></span></strong>:  [<a href="#numpy">NumPy</a>, <a href="#image">Image</a>, <a href="#animation">Animation</a>, <a href="#audio">Audio</a>, <a href="#synthesizer">Synthesizer</a>, <a href="#pygame">Pygame</a>, <a href="#pandas">Pandas</a>, <a href="#plotly">Plotly</a>]
}
</code></pre></div>





<div><h2 id="main"><a href="#main" name="main">#</a>Main</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:      <span class="hljs-comment"># Skips indented lines of code if file was imported.</span>
    main()                      <span class="hljs-comment"># Executes user-defined `def main(): ...` function.</span>
</code></pre></div>

<div><h2 id="list"><a href="#list" name="list">#</a>List</h2><pre><code class="python language-python hljs">&lt;list&gt; = [&lt;el_1&gt;, &lt;el_2&gt;, ...]  <span class="hljs-comment"># Creates new list object. E.g. `list_a = [1, 2, 3]`.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;el&gt;   = &lt;list&gt;[index]          <span class="hljs-comment"># First index is 0, last -1. Also `&lt;list&gt;[i] = &lt;el&gt;`.</span>
&lt;list&gt; = &lt;list&gt;[&lt;slice&gt;]        <span class="hljs-comment"># Also &lt;list&gt;[from_inclusive : to_exclusive : ±step].</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt;.append(&lt;el&gt;)             <span class="hljs-comment"># Appends element to the end. Also `&lt;list&gt; += [&lt;el&gt;]`.</span>
&lt;list&gt;.extend(&lt;collection&gt;)     <span class="hljs-comment"># Appends multiple elements. Also `&lt;list&gt; += &lt;coll&gt;`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt;.sort(reverse=<span class="hljs-keyword">False</span>)      <span class="hljs-comment"># Sorts the elements of the list in ascending order.</span>
&lt;list&gt;.reverse()                <span class="hljs-comment"># Reverses the order of elements. Takes linear time.</span>
&lt;list&gt; = sorted(&lt;collection&gt;)   <span class="hljs-comment"># Returns a new sorted list. Accepts `reverse=True`.</span>
&lt;iter&gt; = reversed(&lt;list&gt;)       <span class="hljs-comment"># Returns reversed iterator. Also list(&lt;iterator&gt;).</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;el&gt;  = max(&lt;collection&gt;)       <span class="hljs-comment"># Returns the largest element. Also min(&lt;el_1&gt;, ...).</span>
&lt;num&gt; = sum(&lt;collection&gt;)       <span class="hljs-comment"># Returns a sum of elements. Also math.prod(&lt;coll&gt;).</span>
</code></pre>
<pre><code class="python language-python hljs">elementwise_sum  = [sum(pair) <span class="hljs-keyword">for</span> pair <span class="hljs-keyword">in</span> zip(list_a, list_b)]
sorted_by_second = sorted(&lt;collection&gt;, key=<span class="hljs-keyword">lambda</span> el: el[<span class="hljs-number">1</span>])
sorted_by_both   = sorted(&lt;collection&gt;, key=<span class="hljs-keyword">lambda</span> el: (el[<span class="hljs-number">1</span>], el[<span class="hljs-number">0</span>]))
flatter_list     = list(itertools.chain.from_iterable(&lt;list&gt;))
</code></pre>
<ul>
<li><strong>For details about sort(), sorted(), min() and max() see <a href="#sortable">Sortable</a>.</strong></li>
<li><strong>Module <a href="#operator">operator</a> has function itemgetter() that can replace listed <a href="#lambda">lambdas</a>.</strong></li>
<li><strong>This text uses the term collection instead of iterable. For rationale see <a href="#collection">Collection</a>.</strong></li>
</ul>
<pre><code class="python language-python hljs">&lt;int&gt; = len(&lt;list/dict/set/…&gt;)  <span class="hljs-comment"># Returns number of items. Doesn't accept iterators.</span>
&lt;int&gt; = &lt;list&gt;.count(&lt;el&gt;)      <span class="hljs-comment"># Counts occurrences. Also `if &lt;el&gt; in &lt;coll&gt;: ...`.</span>
&lt;int&gt; = &lt;list&gt;.index(&lt;el&gt;)      <span class="hljs-comment"># Returns index of first occ. or raises ValueError.</span>
&lt;el&gt;  = &lt;list&gt;.pop()            <span class="hljs-comment"># Removes item from the end (or at index if passed).</span>
&lt;list&gt;.insert(&lt;int&gt;, &lt;el&gt;)      <span class="hljs-comment"># Inserts item at index and shifts remaining items.</span>
&lt;list&gt;.remove(&lt;el&gt;)             <span class="hljs-comment"># Removes the first occurrence or raises ValueError.</span>
&lt;list&gt;.clear()                  <span class="hljs-comment"># Removes all items. Also provided by dict and set.</span>
</code></pre>
<div><h2 id="dictionary"><a href="#dictionary" name="dictionary">#</a>Dictionary</h2><pre><code class="python language-python hljs">&lt;dict&gt; = {key_1: val_1, key_2: val_2, ...}      <span class="hljs-comment"># Use `&lt;dict&gt;[key]` to get or assign the value.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;view&gt; = &lt;dict&gt;.keys()                          <span class="hljs-comment"># A collection of keys that reflects changes.</span>
&lt;view&gt; = &lt;dict&gt;.values()                        <span class="hljs-comment"># A collection of values that reflects changes.</span>
&lt;view&gt; = &lt;dict&gt;.items()                         <span class="hljs-comment"># Coll. of key-value tuples that reflects chgs.</span>
</code></pre>
<pre><code class="python language-python hljs">value  = &lt;dict&gt;.get(key, default=<span class="hljs-keyword">None</span>)          <span class="hljs-comment"># Returns argument default if key is missing.</span>
value  = &lt;dict&gt;.setdefault(key, default=<span class="hljs-keyword">None</span>)   <span class="hljs-comment"># Returns and writes default if key is missing.</span>
&lt;dict&gt; = collections.defaultdict(&lt;type&gt;)        <span class="hljs-comment"># Dict with automatic default value `&lt;type&gt;()`.</span>
&lt;dict&gt; = collections.defaultdict(<span class="hljs-keyword">lambda</span>: <span class="hljs-number">1</span>)     <span class="hljs-comment"># Dictionary with automatic default value 1.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;dict&gt; = dict(&lt;collection&gt;)                     <span class="hljs-comment"># Creates a dict from coll. of key-value pairs.</span>
&lt;dict&gt; = dict(zip(keys, values))                <span class="hljs-comment"># Creates a dictionary from two collections.</span>
&lt;dict&gt; = dict.fromkeys(keys [, value])          <span class="hljs-comment"># Creates a dictionary from collection of keys.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;dict&gt;.update(&lt;dict&gt;)                           <span class="hljs-comment"># Adds items. Replaces ones with matching keys.</span>
value = &lt;dict&gt;.pop(key)                         <span class="hljs-comment"># Removes item or raises KeyError if missing.</span>
{k <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> &lt;dict&gt;.items() <span class="hljs-keyword">if</span> v == value}    <span class="hljs-comment"># Returns set of keys that point to the value.</span>
{k: v <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> &lt;dict&gt;.items() <span class="hljs-keyword">if</span> k <span class="hljs-keyword">in</span> keys}  <span class="hljs-comment"># Returns a dict of items with specified keys.</span>
</code></pre>
<div><h3 id="counter">Counter</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> Counter
<span class="hljs-meta">&gt;&gt;&gt; </span>counter = Counter([<span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'blue'</span>, <span class="hljs-string">'red'</span>, <span class="hljs-string">'red'</span>])
<span class="hljs-meta">&gt;&gt;&gt; </span>counter[<span class="hljs-string">'yellow'</span>] += <span class="hljs-number">1</span>
<span class="hljs-meta">&gt;&gt;&gt; </span>print(counter.most_common())
[(<span class="hljs-string">'blue'</span>, <span class="hljs-number">3</span>), (<span class="hljs-string">'red'</span>, <span class="hljs-number">2</span>), (<span class="hljs-string">'yellow'</span>, <span class="hljs-number">1</span>)]
</code></pre></div>

<div><h2 id="set"><a href="#set" name="set">#</a>Set</h2><pre><code class="python language-python hljs">&lt;set&gt; = {&lt;el_1&gt;, &lt;el_2&gt;, ...}                <span class="hljs-comment"># Coll. of unique items. Also set(), set(&lt;coll&gt;).</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;set&gt;.add(&lt;el&gt;)                              <span class="hljs-comment"># Adds item to the set. Same as `&lt;set&gt; |= {&lt;el&gt;}`.</span>
&lt;set&gt;.update(&lt;collection&gt; [, ...])           <span class="hljs-comment"># Adds items to the set. Same as `&lt;set&gt; |= &lt;set&gt;`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;set&gt;  = &lt;set&gt;.union(&lt;coll&gt;)                 <span class="hljs-comment"># Returns a set of all items. Also &lt;set&gt; | &lt;set&gt;.</span>
&lt;set&gt;  = &lt;set&gt;.intersection(&lt;coll&gt;)          <span class="hljs-comment"># Returns all shared items. Also &lt;set&gt; &amp; &lt;set&gt;.</span>
&lt;set&gt;  = &lt;set&gt;.difference(&lt;coll&gt;)            <span class="hljs-comment"># Returns set's unique items. Also &lt;set&gt; - &lt;set&gt;.</span>
&lt;set&gt;  = &lt;set&gt;.symmetric_difference(&lt;coll&gt;)  <span class="hljs-comment"># Returns non-shared items. Also &lt;set&gt; ^ &lt;set&gt;.</span>
&lt;bool&gt; = &lt;set&gt;.issuperset(&lt;coll&gt;)            <span class="hljs-comment"># Returns False if collection has unique items.</span>
&lt;bool&gt; = &lt;set&gt;.issubset(&lt;coll&gt;)              <span class="hljs-comment"># Is collection a superset? Also &lt;set&gt; &lt;= &lt;set&gt;.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;el&gt; = &lt;set&gt;.pop()                           <span class="hljs-comment"># Removes and returns an item or raises KeyError.</span>
&lt;set&gt;.remove(&lt;el&gt;)                           <span class="hljs-comment"># Removes the item or raises KeyError if missing.</span>
&lt;set&gt;.discard(&lt;el&gt;)                          <span class="hljs-comment"># Same as remove() but it doesn't raise an error.</span>
</code></pre>
<div><h3 id="frozenset">Frozen Set</h3><ul>
<li><strong>Is immutable and hashable.</strong></li>
<li><strong>That means it can be used as a key in a dictionary or as an item in a set.</strong></li>
</ul><pre><code class="python language-python hljs">&lt;frozenset&gt; = frozenset(&lt;collection&gt;)
</code></pre></div>


<div><h2 id="tuple"><a href="#tuple" name="tuple">#</a>Tuple</h2><p><strong>Tuple is an immutable and hashable list.</strong></p><pre><code class="python language-python hljs">&lt;tuple&gt; = ()                        <span class="hljs-comment"># Returns an empty tuple. Also tuple(), tuple(&lt;coll&gt;).</span>
&lt;tuple&gt; = (&lt;el&gt;,)                   <span class="hljs-comment"># Returns a tuple with single element. Same as `&lt;el&gt;,`.</span>
&lt;tuple&gt; = (&lt;el_1&gt;, &lt;el_2&gt; [, ...])  <span class="hljs-comment"># Returns a tuple. Same as `&lt;el_1&gt;, &lt;el_2&gt; [, ...]`.</span>
</code></pre></div>


<div><h3 id="namedtuple">Named Tuple</h3><p><strong>Tuple's subclass with named elements.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
<span class="hljs-meta">&gt;&gt;&gt; </span>Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>p = Point(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>print(p)
Point(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>p.x, p[<span class="hljs-number">1</span>]
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>)
</code></pre></div>


<div><h2 id="range"><a href="#range" name="range">#</a>Range</h2><p><strong>A sequence of evenly spaced integers.</strong></p><pre><code class="python language-python hljs">&lt;range&gt; = range(stop)                <span class="hljs-comment"># I.e. range(to_exclusive). Integers from 0 to `stop-1`.</span>
&lt;range&gt; = range(start, stop)         <span class="hljs-comment"># I.e. range(from_inc, to_exc). From start to `stop-1`.</span>
&lt;range&gt; = range(start, stop, ±step)  <span class="hljs-comment"># I.e. range(from_inclusive, to_exclusive, ±step_size).</span>
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>[i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">3</span>)]
[<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>]
</code></pre>
<div><h2 id="enumerate"><a href="#enumerate" name="enumerate">#</a>Enumerate</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">for</span> i, el <span class="hljs-keyword">in</span> enumerate(&lt;coll&gt;, start=<span class="hljs-number">0</span>):   <span class="hljs-comment"># Returns next element and its index on each pass.</span>
    ...
</code></pre></div>

<div><h2 id="iterator"><a href="#iterator" name="iterator">#</a>Iterator</h2><p><strong>Potentially endless stream of elements.</strong></p><pre><code class="python language-python hljs">&lt;iter&gt; = iter(&lt;collection&gt;)                <span class="hljs-comment"># Iterator that returns coll's elements one by one.</span>
&lt;iter&gt; = iter(&lt;func&gt;, to_exc)              <span class="hljs-comment"># Calls `&lt;func&gt;()` until it returns 'to_exc' value.</span>
&lt;iter&gt; = (&lt;expr&gt; <span class="hljs-keyword">for</span> &lt;name&gt; <span class="hljs-keyword">in</span> &lt;coll&gt;)     <span class="hljs-comment"># E.g. `(i+1 for i in range(3))`. Evaluates lazily.</span>
&lt;el&gt;   = next(&lt;iter&gt;)                      <span class="hljs-comment"># Returns next element. Raises StopIteration on end.</span>
&lt;list&gt; = list(&lt;iter&gt;)                      <span class="hljs-comment"># Returns a list of iterator's remaining elements.</span>
</code></pre></div>


<ul>
<li><strong>For loops call <code class="python hljs"><span class="hljs-string">'iter(&lt;collection&gt;)'</span></code> at the start and <code class="python hljs"><span class="hljs-string">'next(&lt;iter&gt;)'</span></code> on each pass.</strong></li>
<li><strong>Calling <code class="python hljs"><span class="hljs-string">'iter(&lt;iter&gt;)'</span></code> returns unmodified iterator. For details see <a href="#iterator-1">Iterator</a> duck type.</strong></li>
</ul>
<pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre>
<pre><code class="python language-python hljs">&lt;iter&gt; = it.count(start=<span class="hljs-number">0</span>, step=<span class="hljs-number">1</span>)         <span class="hljs-comment"># Returns updated value endlessly. Accepts floats.</span>
&lt;iter&gt; = it.repeat(&lt;el&gt; [, times])         <span class="hljs-comment"># Returns passed element endlessly or 'times' times.</span>
&lt;iter&gt; = it.cycle(&lt;collection&gt;)            <span class="hljs-comment"># Repeats the passed sequence of elements endlessly.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;iter&gt; = it.chain(&lt;coll&gt;, &lt;coll&gt; [, ...])  <span class="hljs-comment"># Empties collections in order (only figuratively).</span>
&lt;iter&gt; = it.chain.from_iterable(&lt;coll&gt;)    <span class="hljs-comment"># Empties collections inside a collection in order.</span>
&lt;iter&gt; = it.islice(&lt;coll&gt;, [start,] stop)  <span class="hljs-comment"># Also accepts `+step`. Start and stop can be None.</span>
&lt;iter&gt; = it.product(&lt;coll&gt;, ...)           <span class="hljs-comment"># Same as `((a, b) for a in arg_1 for b in arg_2)`.</span>
</code></pre>
<div><h2 id="generator"><a href="#generator" name="generator">#</a>Generator</h2><ul>
<li><strong>Any function that contains a yield statement returns a generator.</strong></li>
<li><strong>Generators and iterators are interchangeable.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">count</span><span class="hljs-params">(start, step)</span>:</span>
    <span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
        <span class="hljs-keyword">yield</span> start
        start += step
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>counter = count(<span class="hljs-number">10</span>, <span class="hljs-number">2</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>next(counter), next(counter), next(counter)
(<span class="hljs-number">10</span>, <span class="hljs-number">12</span>, <span class="hljs-number">14</span>)
</code></pre>
<div><h2 id="type"><a href="#type" name="type">#</a>Type</h2><ul>
<li><strong>Everything in Python is an object.</strong></li>
<li><strong>Every object has a certain type.</strong></li>
<li><strong>Type and class are synonymous.</strong></li>
</ul><pre><code class="python language-python hljs">&lt;type&gt; = type(&lt;el&gt;)                <span class="hljs-comment"># Returns object's type. Same as `&lt;el&gt;.__class__`.</span>
&lt;bool&gt; = isinstance(&lt;el&gt;, &lt;type&gt;)  <span class="hljs-comment"># Same result as `issubclass(type(&lt;el&gt;), &lt;type&gt;)`.</span>
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>type(<span class="hljs-string">'a'</span>), <span class="hljs-string">'a'</span>.__class__, str
(&lt;<span class="hljs-class"><span class="hljs-title">class</span> '<span class="hljs-title">str</span>'&gt;, &lt;<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'&gt;, &lt;<span class="hljs-title">class</span> '<span class="hljs-title">str</span>'&gt;)
</span></code></pre>
<div><h4 id="sometypesdonothavebuiltinnamessotheymustbeimported">Some types do not have built-in names, so they must be imported:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> types <span class="hljs-keyword">import</span> FunctionType, MethodType, LambdaType, GeneratorType, ModuleType
</code></pre></div>

<div><h3 id="abstractbaseclasses">Abstract Base Classes</h3><p><strong>Each abstract base class specifies a set of virtual subclasses. These classes are then recognized by isinstance() and issubclass() as subclasses of the ABC, although they are really not. ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods the class has implemented. For instance, Iterable ABC looks for method iter(), while Collection ABC looks for iter(), contains() and len().</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> collections.abc <span class="hljs-keyword">import</span> Iterable, Collection, Sequence
<span class="hljs-meta">&gt;&gt;&gt; </span>isinstance([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>], Iterable)
<span class="hljs-keyword">True</span>
</code></pre></div>


<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┓
┃                  │  Iterable  │ Collection │  Sequence  ┃
┠──────────────────┼────────────┼────────────┼────────────┨
┃ list, range, str │     ✓      │     ✓      │     ✓      ┃
┃ dict, set        │     ✓      │     ✓      │            ┃
┃ iter             │     ✓      │            │            ┃
┗━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┛
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> numbers <span class="hljs-keyword">import</span> Number, Complex, Real, Rational, Integral
<span class="hljs-meta">&gt;&gt;&gt; </span>isinstance(<span class="hljs-number">123</span>, Number)
<span class="hljs-keyword">True</span>
</code></pre>
<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━┓
┃                    │   Number  │  Complex  │   Real   │ Rational │ Integral ┃
┠────────────────────┼───────────┼───────────┼──────────┼──────────┼──────────┨
┃ int                │     ✓     │     ✓     │    ✓     │    ✓     │    ✓     ┃
┃ fractions.Fraction │     ✓     │     ✓     │    ✓     │    ✓     │          ┃
┃ float              │     ✓     │     ✓     │    ✓     │          │          ┃
┃ complex            │     ✓     │     ✓     │          │          │          ┃
┃ decimal.Decimal    │     ✓     │           │          │          │          ┃
┗━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━┛
</code></pre>
<div><h2 id="string"><a href="#string" name="string">#</a>String</h2><p><strong>Immutable sequence of characters.</strong></p><pre><code class="python language-python hljs">&lt;str&gt;  = <span class="hljs-string">'abc'</span>                               <span class="hljs-comment"># Also "abc". Interprets \n, \t, \x00-\xff, etc.</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;str&gt;  = &lt;str&gt;.strip()                       <span class="hljs-comment"># Strips all whitespace characters from both ends.</span>
&lt;str&gt;  = &lt;str&gt;.strip(<span class="hljs-string">'&lt;chars&gt;'</span>)              <span class="hljs-comment"># Strips passed characters. Also lstrip/rstrip().</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt; = &lt;str&gt;.split()                       <span class="hljs-comment"># Splits it on one or more whitespace characters.</span>
&lt;list&gt; = &lt;str&gt;.split(sep=<span class="hljs-keyword">None</span>, maxsplit=<span class="hljs-number">-1</span>)  <span class="hljs-comment"># Splits on 'sep' string at most 'maxsplit' times.</span>
&lt;list&gt; = &lt;str&gt;.splitlines(keepends=<span class="hljs-keyword">False</span>)    <span class="hljs-comment"># On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n.</span>
&lt;str&gt;  = &lt;str&gt;.join(&lt;coll_of_strings&gt;)       <span class="hljs-comment"># Joins items by using the string as a separator.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;bool&gt; = &lt;sub_str&gt; <span class="hljs-keyword">in</span> &lt;str&gt;                  <span class="hljs-comment"># Returns True if string contains the substring.</span>
&lt;bool&gt; = &lt;str&gt;.startswith(&lt;sub_str&gt;)         <span class="hljs-comment"># Pass tuple of strings to give multiple options.</span>
&lt;int&gt;  = &lt;str&gt;.find(&lt;sub_str&gt;)               <span class="hljs-comment"># Returns start index of the first match or -1.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;str&gt;  = &lt;str&gt;.lower()                       <span class="hljs-comment"># Lowers the case. Also upper/capitalize/title().</span>
&lt;str&gt;  = &lt;str&gt;.casefold()                    <span class="hljs-comment"># Lower() that converts ẞ/ß to ss, Σ/ς to σ, etc.</span>
&lt;str&gt;  = &lt;str&gt;.replace(old, new [, count])   <span class="hljs-comment"># Replaces 'old' with 'new' at most 'count' times.</span>
&lt;str&gt;  = &lt;str&gt;.translate(table)              <span class="hljs-comment"># Use `str.maketrans(&lt;dict&gt;)` to generate table.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;str&gt;  = chr(&lt;int&gt;)                          <span class="hljs-comment"># Converts passed integer into Unicode character.</span>
&lt;int&gt;  = ord(&lt;str&gt;)                          <span class="hljs-comment"># Converts passed Unicode character into integer.</span>
</code></pre>
<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'unicodedata.normalize("NFC", &lt;str&gt;)'</span></code> on strings like <code class="python hljs"><span class="hljs-string">'Motörhead'</span></code> before comparing them to other strings, because <code class="python hljs"><span class="hljs-string">'ö'</span></code> can be stored as one or two characters.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'NFC'</span></code> converts such characters to a single character, while <code class="python hljs"><span class="hljs-string">'NFD'</span></code> converts them to two.</strong></li>
</ul>
<pre><code class="python language-python hljs">&lt;bool&gt; = &lt;str&gt;.isdecimal()                   <span class="hljs-comment"># Checks all chars for [0-9]. Also [०-९], [٠-٩].</span>
&lt;bool&gt; = &lt;str&gt;.isdigit()                     <span class="hljs-comment"># Checks for [²³¹…] and isdecimal(). Also [፩-፱].</span>
&lt;bool&gt; = &lt;str&gt;.isnumeric()                   <span class="hljs-comment"># Checks for [¼½¾…] and isdigit(). Also [零〇一…].</span>
&lt;bool&gt; = &lt;str&gt;.isalnum()                     <span class="hljs-comment"># Checks for [ABC…] and isnumeric(). Also [ªµº…].</span>
&lt;bool&gt; = &lt;str&gt;.isprintable()                 <span class="hljs-comment"># Checks for [ !"#$…] and isalnum(). Also emojis.</span>
&lt;bool&gt; = &lt;str&gt;.isspace()                     <span class="hljs-comment"># Checks for [ \t\n\r\f\v\x1c\x1d\x1e\x1f\x85…].</span>
</code></pre>
<div><h2 id="regex"><a href="#regex" name="regex">#</a>Regex</h2><p><strong>Functions for regular expression matching.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> re
&lt;str&gt;   = re.sub(<span class="hljs-string">r'&lt;regex&gt;'</span>, new, text, count=<span class="hljs-number">0</span>)  <span class="hljs-comment"># Substitutes occurrences with string 'new'.</span>
&lt;list&gt;  = re.findall(<span class="hljs-string">r'&lt;regex&gt;'</span>, text)            <span class="hljs-comment"># Returns every occurrence of the pattern.</span>
&lt;list&gt;  = re.split(<span class="hljs-string">r'&lt;regex&gt;'</span>, text, maxsplit=<span class="hljs-number">0</span>)  <span class="hljs-comment"># Add brackets around regex to keep matches.</span>
&lt;Match&gt; = re.search(<span class="hljs-string">r'&lt;regex&gt;'</span>, text)             <span class="hljs-comment"># Returns first occ. of the pattern or None.</span>
&lt;Match&gt; = re.match(<span class="hljs-string">r'&lt;regex&gt;'</span>, text)              <span class="hljs-comment"># Only searches at the start of the 'text'.</span>
&lt;iter&gt;  = re.finditer(<span class="hljs-string">r'&lt;regex&gt;'</span>, text)           <span class="hljs-comment"># Returns all occurrences as Match objects.</span>
</code></pre></div>


<ul>
<li><strong>Raw string literals do not interpret escape sequences, thus enabling us to use regex-specific escape sequences that cause SyntaxWarning in normal string literals (since 3.12).</strong></li>
<li><strong>Argument 'new' of re.sub() can be a function that accepts Match object and returns a str.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.IGNORECASE'</span></code> can be used with all functions that are listed above.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.MULTILINE'</span></code> makes <code class="python hljs"><span class="hljs-string">'^'</span></code> and <code class="python hljs"><span class="hljs-string">'$'</span></code> match the start/end of each line.</strong></li>
<li><strong>Argument <code class="python hljs"><span class="hljs-string">'flags=re.DOTALL'</span></code> makes <code class="python hljs"><span class="hljs-string">'.'</span></code> also accept the <code class="python hljs"><span class="hljs-string">'\n'</span></code> (besides all other chars).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'re.compile("&lt;regex&gt;")'</span></code> returns a Pattern object with methods sub(), findall(), etc.</strong></li>
</ul>
<div><h3 id="matchobject">Match Object</h3><pre><code class="python language-python hljs">&lt;str&gt;   = &lt;Match&gt;.group()                         <span class="hljs-comment"># Returns the whole match. Also group(0).</span>
&lt;str&gt;   = &lt;Match&gt;.group(<span class="hljs-number">1</span>)                        <span class="hljs-comment"># Returns part inside the first brackets.</span>
&lt;tuple&gt; = &lt;Match&gt;.groups()                        <span class="hljs-comment"># Returns all bracketed parts as strings.</span>
&lt;int&gt;   = &lt;Match&gt;.start()                         <span class="hljs-comment"># Returns start index of the whole match.</span>
&lt;int&gt;   = &lt;Match&gt;.end()                           <span class="hljs-comment"># Returns a match's exclusive end index.</span>
</code></pre></div>

<div><h3 id="specialsequences">Special Sequences</h3><pre><code class="python language-python hljs"><span class="hljs-string">'\d'</span> == <span class="hljs-string">'[0-9]'</span>                                   <span class="hljs-comment"># Also [०-९…]. Matches decimal character.</span>
<span class="hljs-string">'\w'</span> == <span class="hljs-string">'[a-zA-Z0-9_]'</span>                            <span class="hljs-comment"># Also [ª²³…]. Matches alphanumeric or _.</span>
<span class="hljs-string">'\s'</span> == <span class="hljs-string">'[ \t\n\r\f\v]'</span>                           <span class="hljs-comment"># Also [\x1c-\x1f…]. Matches whitespace.</span>
</code></pre></div>

<ul>
<li><strong>By default, decimal characters and alphanumerics from all alphabets are matched unless <code class="python hljs"><span class="hljs-string">'flags=re.ASCII'</span></code> is used. It restricts special sequence matches to the first 128 Unicode characters and also prevents <code class="python hljs"><span class="hljs-string">'\s'</span></code> from accepting <code class="python hljs"><span class="hljs-string">'\x1c'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1d'</span></code>, <code class="python hljs"><span class="hljs-string">'\x1e'</span></code> and <code class="python hljs"><span class="hljs-string">'\x1f'</span></code> (non-printable characters that divide text into files, tables, rows and fields, respectively).</strong></li>
<li><strong>Use a capital letter for negation (all non-ASCII characters will be matched when used in combination with ASCII flag).</strong></li>
</ul>
<div><h2 id="format"><a href="#format" name="format">#</a>Format</h2><pre><code class="python hljs">&lt;str&gt; = <span class="hljs-string">f'<span class="hljs-subst">{&lt;el_1&gt;}</span>, <span class="hljs-subst">{&lt;el_2&gt;}</span>'</span>            <span class="hljs-comment"># Curly brackets can also contain expressions.</span>
&lt;str&gt; = <span class="hljs-string">'{}, {}'</span>.format(&lt;el_1&gt;, &lt;el_2&gt;)  <span class="hljs-comment"># Same as '{0}, {a}'.format(&lt;el_1&gt;, a=&lt;el_2&gt;).</span>
&lt;str&gt; = <span class="hljs-string">'%s, %s'</span> % (&lt;el_1&gt;, &lt;el_2&gt;)      <span class="hljs-comment"># Redundant and inferior C-style formatting.</span>
</code></pre></div>

<div><h3 id="example">Example</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>Person = collections.namedtuple(<span class="hljs-string">'Person'</span>, <span class="hljs-string">'name height'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>person = Person(<span class="hljs-string">'Jean-Luc'</span>, <span class="hljs-number">187</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-string">f'<span class="hljs-subst">{person.name}</span> is <span class="hljs-subst">{person.height / <span class="hljs-number">100</span>}</span> meters tall.'</span>
<span class="hljs-string">'Jean-Luc is 1.87 meters tall.'</span>
</code></pre></div>

<div><h3 id="generaloptions">General Options</h3><pre><code class="python language-python hljs">{&lt;el&gt;:&lt;<span class="hljs-number">10</span>}                               <span class="hljs-comment"># '&lt;el&gt;      '</span>
{&lt;el&gt;:^<span class="hljs-number">10</span>}                               <span class="hljs-comment"># '   &lt;el&gt;   '</span>
{&lt;el&gt;:&gt;<span class="hljs-number">10</span>}                               <span class="hljs-comment"># '      &lt;el&gt;'</span>
{&lt;el&gt;:.&lt;<span class="hljs-number">10</span>}                              <span class="hljs-comment"># '&lt;el&gt;......'</span>
{&lt;el&gt;:<span class="hljs-number">0</span>}                                 <span class="hljs-comment"># '&lt;el&gt;'</span>
</code></pre></div>

<ul>
<li><strong>Objects are rendered by calling the format() function, e.g. <code class="python hljs"><span class="hljs-string">'format(&lt;el&gt;, "&lt;10")'</span></code>.</strong></li>
<li><strong>Options can be generated dynamically via nested braces: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{&lt;el&gt;:{&lt;str/int&gt;}</span>[…]}'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'='</span></code> to the expression prepends it to the output: <code class="python hljs"><span class="hljs-string">f'<span class="hljs-subst">{<span class="hljs-number">1</span>+<span class="hljs-number">1</span>=}</span>'</span></code> returns <code class="python hljs"><span class="hljs-string">'1+1=2'</span></code>.</strong></li>
<li><strong>Adding <code class="python hljs"><span class="hljs-string">'!r'</span></code> to the expression converts object to string by calling its <a href="#class">repr()</a> method.</strong></li>
</ul>
<div><h3 id="strings">Strings</h3><pre><code class="python language-python hljs">{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10</span>}                             <span class="hljs-comment"># 'abcde     '</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">10.3</span>}                           <span class="hljs-comment"># 'abc       '</span>
{<span class="hljs-string">'abcde'</span>:<span class="hljs-number">.3</span>}                             <span class="hljs-comment"># 'abc'</span>
{<span class="hljs-string">'abcde'</span>!r:<span class="hljs-number">10</span>}                           <span class="hljs-comment"># "'abcde'   "</span>
</code></pre></div>

<div><h3 id="numbers-1">Numbers</h3><pre><code class="python language-python hljs">{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>}                              <span class="hljs-comment"># '    123456'</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>,}                             <span class="hljs-comment"># '   123,456'</span>
{<span class="hljs-number">123456</span>:<span class="hljs-number">10</span>_}                             <span class="hljs-comment"># '   123_456'</span>
{<span class="hljs-number">123456</span>:+<span class="hljs-number">10</span>}                             <span class="hljs-comment"># '   +123456'</span>
{<span class="hljs-number">123456</span>:=+<span class="hljs-number">10</span>}                            <span class="hljs-comment"># '+   123456'</span>
{<span class="hljs-number">123456</span>: }                               <span class="hljs-comment"># ' 123456'</span>
{<span class="hljs-number">-123456</span>: }                              <span class="hljs-comment"># '-123456'</span>
</code></pre></div>

<div><h3 id="floats">Floats</h3><pre><code class="python language-python hljs">{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>}                           <span class="hljs-comment"># '      1.23'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>f}                          <span class="hljs-comment"># '     1.235'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>e}                          <span class="hljs-comment"># ' 1.235e+00'</span>
{<span class="hljs-number">1.23456</span>:<span class="hljs-number">10.3</span>%}                          <span class="hljs-comment"># '  123.456%'</span>
</code></pre></div>

<div><h4 id="comparisonofpresentationtypes">Comparison of presentation types:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃              │    {&lt;float&gt;}   │   {&lt;float&gt;:f}  │   {&lt;float&gt;:e}  │   {&lt;float&gt;:%}  ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃  0.000056789 │   '5.6789e-05' │    '0.000057'  │ '5.678900e-05' │    '0.005679%' ┃
┃  0.00056789  │   '0.00056789' │    '0.000568'  │ '5.678900e-04' │    '0.056789%' ┃
┃  0.0056789   │   '0.0056789'  │    '0.005679'  │ '5.678900e-03' │    '0.567890%' ┃
┃  0.056789    │   '0.056789'   │    '0.056789'  │ '5.678900e-02' │    '5.678900%' ┃
┃  0.56789     │   '0.56789'    │    '0.567890'  │ '5.678900e-01' │   '56.789000%' ┃
┃  5.6789      │   '5.6789'     │    '5.678900'  │ '5.678900e+00' │  '567.890000%' ┃
┃ 56.789       │  '56.789'      │   '56.789000'  │ '5.678900e+01' │ '5678.900000%' ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛

┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃              │  {&lt;float&gt;:.2}  │  {&lt;float&gt;:.2f} │  {&lt;float&gt;:.2e} │  {&lt;float&gt;:.2%} ┃
┠──────────────┼────────────────┼────────────────┼────────────────┼────────────────┨
┃  0.000056789 │    '5.7e-05'   │      '0.00'    │   '5.68e-05'   │      '0.01%'   ┃
┃  0.00056789  │    '0.00057'   │      '0.00'    │   '5.68e-04'   │      '0.06%'   ┃
┃  0.0056789   │    '0.0057'    │      '0.01'    │   '5.68e-03'   │      '0.57%'   ┃
┃  0.056789    │    '0.057'     │      '0.06'    │   '5.68e-02'   │      '5.68%'   ┃
┃  0.56789     │    '0.57'      │      '0.57'    │   '5.68e-01'   │     '56.79%'   ┃
┃  5.6789      │    '5.7'       │      '5.68'    │   '5.68e+00'   │    '567.89%'   ┃
┃ 56.789       │    '5.7e+01'   │     '56.79'    │   '5.68e+01'   │   '5678.90%'   ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
</code></pre></div>


<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'{&lt;float&gt;:g}'</span></code> is <code class="python hljs"><span class="hljs-string">'{&lt;float&gt;:.6}'</span></code> with stripped zeros, exponent starting at <code class="python hljs"><span class="hljs-string">'1e+06'</span></code>.</strong></li>
<li><strong>When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes <code class="python hljs"><span class="hljs-string">'{6.5:.0f}'</span></code> a <code class="python hljs"><span class="hljs-string">'6'</span></code> and <code class="python hljs"><span class="hljs-string">'{7.5:.0f}'</span></code> an <code class="python hljs"><span class="hljs-string">'8'</span></code>.</strong></li>
<li><strong>This rule only effects numbers that can be represented exactly by a float (<code class="python hljs"><span class="hljs-number">.5</span></code>, <code class="python hljs"><span class="hljs-number">.25</span></code>, …).</strong></li>
</ul>
<div><h3 id="ints">Ints</h3><pre><code class="python language-python hljs">{<span class="hljs-number">90</span>:c}                                   <span class="hljs-comment"># Converts 90 to Unicode character 'Z'.</span>
{<span class="hljs-number">90</span>:b}                                   <span class="hljs-comment"># Converts 90 to binary number '1011010'.</span>
{<span class="hljs-number">90</span>:X}                                   <span class="hljs-comment"># Converts 90 to hexadecimal number '5A'.</span>
</code></pre></div>

<div><h2 id="numbers"><a href="#numbers" name="numbers">#</a>Numbers</h2><pre><code class="python language-python hljs">&lt;int&gt;      = int(&lt;float/str/bool&gt;)             <span class="hljs-comment"># A whole number. Floats get truncated.</span>
&lt;float&gt;    = float(&lt;int/str/bool&gt;)             <span class="hljs-comment"># A 64-bit decimal. Also &lt;float&gt;e±&lt;int&gt;.</span>
&lt;complex&gt;  = complex(real=<span class="hljs-number">0</span>, imag=<span class="hljs-number">0</span>)           <span class="hljs-comment"># Complex number. Also &lt;float&gt;±&lt;float&gt;j.</span>
&lt;Fraction&gt; = fractions.Fraction(&lt;int&gt;, &lt;int&gt;)  <span class="hljs-comment"># `Fraction(1, 2) / 3 == Fraction(1, 6)`.</span>
&lt;Decimal&gt;  = decimal.Decimal(&lt;str/int/tuple&gt;)  <span class="hljs-comment"># `Decimal((1, (2, 3), 4)) == -230_000`.</span>
</code></pre></div>

<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'int(&lt;str&gt;)'</span></code> and <code class="python hljs"><span class="hljs-string">'float(&lt;str&gt;)'</span></code> raise ValueError exception if string is malformed.</strong></li>
<li><strong>Decimal objects store numbers exactly, unlike most floats where <code class="python hljs"><span class="hljs-string">'1.1 + 2.2 != 3.3'</span></code>.</strong></li>
<li><strong>Floats can be compared with: <code class="python hljs"><span class="hljs-string">'math.isclose(&lt;float&gt;, &lt;float&gt;, rel_tol=1e-09)'</span></code>.</strong></li>
<li><strong>Precision of decimal operations is set with: <code class="python hljs"><span class="hljs-string">'decimal.getcontext().prec = &lt;int&gt;'</span></code>.</strong></li>
<li><strong>Bools can be used anywhere ints can, because bool is a subclass of int: <code class="python hljs"><span class="hljs-string">'True + 1 == 2'</span></code>.</strong></li>
</ul>
<div><h3 id="builtinfunctions">Built-in Functions</h3><pre><code class="python language-python hljs">&lt;num&gt; = pow(&lt;num&gt;, &lt;num&gt;)                      <span class="hljs-comment"># E.g. `pow(2, 3) == 2 ** 3 == 8`.</span>
&lt;num&gt; = abs(&lt;num&gt;)                             <span class="hljs-comment"># E.g. `abs(complex(3, 4)) == 5`.</span>
&lt;num&gt; = round(&lt;num&gt; [, ±ndigits])              <span class="hljs-comment"># E.g. `round(123.45, -1) == 120`.</span>
&lt;num&gt; = min(&lt;coll_of_nums&gt;)                    <span class="hljs-comment"># Also max(&lt;num&gt;, &lt;num&gt; [, ...]).</span>
&lt;num&gt; = sum(&lt;coll_of_nums&gt;)                    <span class="hljs-comment"># Also math.prod(&lt;coll_of_nums&gt;).</span>
</code></pre></div>

<div><h3 id="math">Math</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> floor, ceil, trunc            <span class="hljs-comment"># Funcs that convert floats to ints.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> pi, inf, nan, isnan           <span class="hljs-comment"># `inf*0` and `nan+1` are both nan.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sqrt, factorial               <span class="hljs-comment"># `sqrt(-1)` raises ValueError excp.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> sin, cos, tan                 <span class="hljs-comment"># Also: degrees, radians, asin, etc.</span>
<span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> log, log10, log2              <span class="hljs-comment"># Base can be passed via second arg.</span>
</code></pre></div>

<div><h3 id="statistics">Statistics</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> statistics <span class="hljs-keyword">import</span> mean, median, mode      <span class="hljs-comment"># Mode returns most common element.</span>
<span class="hljs-keyword">from</span> statistics <span class="hljs-keyword">import</span> variance, stdev         <span class="hljs-comment"># Also `cuts = quantiles(data, n)`.</span>
</code></pre></div>

<div><h3 id="random">Random</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> random, randint, uniform    <span class="hljs-comment"># Also: gauss, choice, shuffle, etc.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;float&gt; = random()                             <span class="hljs-comment"># Selects random float from [0, 1).</span>
&lt;num&gt;   = randint/uniform(a, b)                <span class="hljs-comment"># Selects an int/float from [a, b].</span>
&lt;float&gt; = gauss(mean, stdev)                   <span class="hljs-comment"># Also triangular(low, high, mode).</span>
&lt;el&gt;    = choice(&lt;sequence&gt;)                   <span class="hljs-comment"># Doesn't modify. Also sample(p, n).</span>
shuffle(&lt;list&gt;)                                <span class="hljs-comment"># Works with all mutable sequences.</span>
</code></pre>
<div><h3 id="hexadecimalnumbers">Hexadecimal Numbers</h3><pre><code class="python language-python hljs">&lt;int&gt; = <span class="hljs-number">0x</span>&lt;hex&gt;                                <span class="hljs-comment"># E.g. `0xFf == 255`. Also 0b&lt;bin&gt;.</span>
&lt;int&gt; = int(<span class="hljs-string">'±&lt;hex&gt;'</span>, <span class="hljs-number">16</span>)                      <span class="hljs-comment"># Also int('±0x&lt;hex&gt;/±0b&lt;bin&gt;', 0).</span>
&lt;str&gt; = hex(&lt;int&gt;)                             <span class="hljs-comment"># Returns '[-]0x&lt;hex&gt;'. Also bin().</span>
</code></pre></div>

<div><h3 id="bitwiseoperators">Bitwise Operators</h3><pre><code class="python language-python hljs">&lt;int&gt; = &lt;int&gt; &amp; &lt;int&gt;                          <span class="hljs-comment"># E.g. `0b1100 &amp; 0b1010 == 0b1000`.</span>
&lt;int&gt; = &lt;int&gt; | &lt;int&gt;                          <span class="hljs-comment"># E.g. `0b1100 | 0b1010 == 0b1110`.</span>
&lt;int&gt; = &lt;int&gt; ^ &lt;int&gt;                          <span class="hljs-comment"># E.g. `0b1100 ^ 0b1010 == 0b0110`.</span>
&lt;int&gt; = &lt;int&gt; &lt;&lt; n_bits                        <span class="hljs-comment"># E.g. `0b1111 &lt;&lt; 4 == 0b11110000`.</span>
&lt;int&gt; = ~&lt;int&gt;                                 <span class="hljs-comment"># E.g. `~0b1 == -(0b1+1) == -0b10`.</span>
</code></pre></div>

<div><h2 id="combinatorics"><a href="#combinatorics" name="combinatorics">#</a>Combinatorics</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it
</code></pre></div>

<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>list(it.product(<span class="hljs-string">'abc'</span>, repeat=<span class="hljs-number">2</span>))        <span class="hljs-comment">#   a  b  c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>),         <span class="hljs-comment"># a x  x  x</span>
 (<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>),         <span class="hljs-comment"># b x  x  x</span>
 (<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'c'</span>)]         <span class="hljs-comment"># c x  x  x</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>list(it.permutations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>))          <span class="hljs-comment">#   a  b  c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>),                     <span class="hljs-comment"># a .  x  x</span>
 (<span class="hljs-string">'b'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>),                     <span class="hljs-comment"># b x  .  x</span>
 (<span class="hljs-string">'c'</span>, <span class="hljs-string">'a'</span>), (<span class="hljs-string">'c'</span>, <span class="hljs-string">'b'</span>)]                     <span class="hljs-comment"># c x  x  .</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>list(it.combinations(<span class="hljs-string">'abc'</span>, <span class="hljs-number">2</span>))          <span class="hljs-comment">#   a  b  c</span>
[(<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>), (<span class="hljs-string">'a'</span>, <span class="hljs-string">'c'</span>),                     <span class="hljs-comment"># a .  x  x</span>
 (<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>)                                  <span class="hljs-comment"># b .  .  x</span>
]                                            <span class="hljs-comment"># c .  .  .</span>
</code></pre>
<div><h2 id="datetime"><a href="#datetime" name="datetime">#</a>Datetime</h2><p><strong>Provides 'date', 'time', 'datetime' and 'timedelta' classes. All are immutable and hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install python-dateutil</span>
<span class="hljs-keyword">from</span> datetime <span class="hljs-keyword">import</span> date, time, datetime, timedelta, timezone
<span class="hljs-keyword">import</span> zoneinfo, dateutil.tz
</code></pre></div>


<pre><code class="python language-python apache hljs">&lt;D&gt;  = date(year, month, day)               <span class="hljs-comment"># Accepts valid dates between AD 1 and AD 9999.</span>
&lt;T&gt;  = time(hour=<span class="hljs-number">0</span>, minute=<span class="hljs-number">0</span>, second=<span class="hljs-number">0</span>)     <span class="hljs-comment"># Also: `microsecond=0, tzinfo=None, fold=0`.</span>
&lt;DT&gt; = datetime(year, month, day, hour=<span class="hljs-number">0</span>)   <span class="hljs-comment"># Also: `minute=0, second=0, microsecond=0, …`.</span>
&lt;TD&gt; = timedelta(weeks=<span class="hljs-number">0</span>, days=<span class="hljs-number">0</span>, hours=<span class="hljs-number">0</span>)  <span class="hljs-comment"># Also: `minutes=0, seconds=0, microseconds=0`.</span>
</code></pre>
<ul>
<li><strong>Times and datetimes that have defined timezone are called aware and ones that don't, naive. If time or datetime object is naive, it is presumed to be in the system's timezone!</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fold=1'</span></code> means the second pass in case of time jumping back (usually for one hour).</strong></li>
<li><strong>Timedelta normalizes arguments to ±days, seconds (&lt; 86 400) and microseconds (&lt; 1M). Its str() method returns <code class="python hljs"><span class="hljs-string">'[±D, ]H:MM:SS[.…]'</span></code> and total_seconds() a float of all seconds.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'&lt;D/DT&gt;.weekday()'</span></code> to get the day of the week as an integer, with Monday being 0.</strong></li>
</ul>
<div><h3 id="now">Now</h3><pre><code class="python language-python hljs">&lt;D/DTn&gt; = D/DT.today()                      <span class="hljs-comment"># Current local date or naive DT. Also DT.now().</span>
&lt;DTa&gt;   = DT.now(&lt;tzinfo&gt;)                  <span class="hljs-comment"># Aware DT from current time in passed timezone.</span>
</code></pre></div>

<ul>
<li><strong>To extract time use <code class="python hljs"><span class="hljs-string">'&lt;DTn&gt;.time()'</span></code>, <code class="python hljs"><span class="hljs-string">'&lt;DTa&gt;.time()'</span></code> or <code class="python hljs"><span class="hljs-string">'&lt;DTa&gt;.timetz()'</span></code>.</strong></li>
</ul>
<div><h3 id="timezones">Timezones</h3><pre><code class="python language-python apache hljs">&lt;tzinfo&gt; = timezone.utc                     <span class="hljs-comment"># Coordinated universal time. London without DST.</span>
&lt;tzinfo&gt; = timezone(&lt;timedelta&gt;)            <span class="hljs-comment"># Timezone with fixed offset from universal time.</span>
&lt;tzinfo&gt; = dateutil.tz.tzlocal()            <span class="hljs-comment"># Local timezone with dynamic offset from the UTC.</span>
&lt;tzinfo&gt; = zoneinfo.ZoneInfo(<span class="hljs-string">'&lt;iana_key&gt;'</span>)  <span class="hljs-comment"># 'Continent/City_Name' zone with dynamic offset.</span>
&lt;DTa&gt;    = &lt;DT&gt;.astimezone([&lt;tzinfo&gt;])      <span class="hljs-comment"># Converts DT to the passed or local fixed zone.</span>
&lt;Ta/DTa&gt; = &lt;T/DT&gt;.replace(tzinfo=&lt;tzinfo&gt;)  <span class="hljs-comment"># Changes the timezone object without conversion.</span>
</code></pre></div>

<ul>
<li><strong>Timezones returned by tzlocal(), ZoneInfo() and implicit local timezone of naive objects have offsets that vary through time due to DST and historical changes of the base offset.</strong></li>
<li><strong>To get ZoneInfo() to work on Windows run <code class="python hljs"><span class="hljs-string">'&gt; pip3 install tzdata'</span></code>.</strong></li>
</ul>
<div><h3 id="encode">Encode</h3><pre><code class="python language-python apache hljs">&lt;D/T/DT&gt; = D/T/DT.fromisoformat(&lt;str&gt;)      <span class="hljs-comment"># Object from an ISO string. Raises ValueError.</span>
&lt;DT&gt;     = DT.strptime(&lt;str&gt;, <span class="hljs-string">'&lt;format&gt;'</span>)   <span class="hljs-comment"># Naive or aware datetime from a custom string.</span>
&lt;D/DTn&gt;  = D/DT.fromordinal(&lt;int&gt;)          <span class="hljs-comment"># D or DT from days since the Gregorian NYE 1.</span>
&lt;DTn&gt;    = DT.fromtimestamp(&lt;float&gt;)        <span class="hljs-comment"># Local naive DT from seconds since the Epoch.</span>
&lt;DTa&gt;    = DT.fromtimestamp(&lt;float&gt;, &lt;tz&gt;)  <span class="hljs-comment"># Aware datetime from seconds since the Epoch.</span>
</code></pre></div>

<ul>
<li><strong>ISO strings come in following forms: <code class="python hljs"><span class="hljs-string">'YYYY-MM-DD'</span></code>, <code class="python hljs"><span class="hljs-string">'HH:MM:SS.mmmuuu[±HH:MM]'</span></code>, or both separated by an arbitrary character. All parts following the hours are optional.</strong></li>
<li><strong>Python uses the Unix Epoch: <code class="python hljs"><span class="hljs-string">'1970-01-01 00:00 UTC'</span></code>, <code class="python hljs"><span class="hljs-string">'1970-01-01 01:00 CET'</span></code>, …</strong></li>
</ul>
<div><h3 id="decode">Decode</h3><pre><code class="python language-python hljs">&lt;str&gt;    = &lt;D/T/DT&gt;.isoformat(sep=<span class="hljs-string">'T'</span>)      <span class="hljs-comment"># Also `timespec='auto/hours/minutes/seconds/…'`.</span>
&lt;str&gt;    = &lt;D/T/DT&gt;.strftime(<span class="hljs-string">'&lt;format&gt;'</span>)    <span class="hljs-comment"># Returns custom string representation of object.</span>
&lt;int&gt;    = &lt;D/DT&gt;.toordinal()               <span class="hljs-comment"># Days since NYE 1, ignoring DT's time and zone.</span>
&lt;float&gt;  = &lt;DTn&gt;.timestamp()                <span class="hljs-comment"># Seconds since the Epoch, from local naive DT.</span>
&lt;float&gt;  = &lt;DTa&gt;.timestamp()                <span class="hljs-comment"># Seconds since the Epoch, from aware datetime.</span>
</code></pre></div>

<div><h3 id="format-1">Format</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>dt = datetime.strptime(<span class="hljs-string">'2025-08-14 23:39:00.00 +0200'</span>, <span class="hljs-string">'%Y-%m-%d %H:%M:%S.%f %z'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>dt.strftime(<span class="hljs-string">"%dth of %B '%y (%a), %I:%M %p %Z"</span>)
<span class="hljs-string">"14th of August '25 (Thu), 11:39 PM UTC+02:00"</span>
</code></pre></div>

<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'%z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'±HH[:]MM'</span></code> and returns <code class="python hljs"><span class="hljs-string">'±HHMM'</span></code> or empty string if object is naive.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'%Z'</span></code> accepts <code class="python hljs"><span class="hljs-string">'UTC/GMT'</span></code> and local timezone's code and returns timezone's name, <code class="python hljs"><span class="hljs-string">'UTC[±HH:MM]'</span></code> if timezone is nameless, or an empty string if object is naive.</strong></li>
</ul>
<div><h3 id="arithmetics">Arithmetics</h3><pre><code class="python language-python apache hljs">&lt;bool&gt;   = &lt;D/T/DTn&gt; &gt; &lt;D/T/DTn&gt;            <span class="hljs-comment"># Ignores time jumps (fold attribute). Also `==`.</span>
&lt;bool&gt;   = &lt;DTa&gt;     &gt; &lt;DTa&gt;                <span class="hljs-comment"># Ignores time jumps if they share tzinfo object.</span>
&lt;TD&gt;     = &lt;D/DTn&gt;   - &lt;D/DTn&gt;              <span class="hljs-comment"># Ignores jumps. Convert to UTC for actual delta.</span>
&lt;TD&gt;     = &lt;DTa&gt;     - &lt;DTa&gt;                <span class="hljs-comment"># Ignores jumps if they share the tzinfo object.</span>
&lt;D/DT&gt;   = &lt;D/DT&gt;    ± &lt;TD&gt;                 <span class="hljs-comment"># Returned datetime can fall into a missing hour.</span>
&lt;TD&gt;     = &lt;TD&gt;      * &lt;float&gt;              <span class="hljs-comment"># Also `&lt;TD&gt; = &lt;TD&gt; ± &lt;TD&gt;`, `&lt;TD&gt; = abs(&lt;TD&gt;)`.</span>
&lt;float&gt;  = &lt;TD&gt;      / &lt;TD&gt;                 <span class="hljs-comment"># Calling divmod(&lt;TD&gt;, &lt;TD&gt;) returns int and TD.</span>
</code></pre></div>

<div><h2 id="function"><a href="#function" name="function">#</a>Function</h2><p><strong>Independent block of code that returns a value when called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> &lt;<span class="hljs-title">func_name</span>&gt;<span class="hljs-params">(&lt;nondefault_args&gt;)</span>:</span> ...                  <span class="hljs-comment"># E.g. `def func(x, y): ...`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> &lt;<span class="hljs-title">func_name</span>&gt;<span class="hljs-params">(&lt;default_args&gt;)</span>:</span> ...                     <span class="hljs-comment"># E.g. `def func(x=0, y=0): ...`.</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> &lt;<span class="hljs-title">func_name</span>&gt;<span class="hljs-params">(&lt;nondefault_args&gt;, &lt;default_args&gt;)</span>:</span> ...  <span class="hljs-comment"># E.g. `def func(x, y=0): ...`.</span>
</code></pre></div>


<ul>
<li><strong>Function returns None if it doesn't encounter the <code class="python hljs"><span class="hljs-string">'return &lt;object/expr&gt;'</span></code> statement.</strong></li>
<li><strong>Run <code class="python hljs"><span class="hljs-string">'global &lt;var_name&gt;'</span></code> inside the function before assigning to the global variable.</strong></li>
<li><strong>Value of a default argument is evaluated when function is first encountered in the scope.</strong></li>
<li><strong>Any mutation of a default argument value will persist between function invocations!</strong></li>
</ul>
<div><h3 id="functioncall">Function Call</h3><pre><code class="python language-python hljs">&lt;obj&gt; = &lt;function&gt;(&lt;positional_args&gt;)                    <span class="hljs-comment"># E.g. `func(0, 0)`.</span>
&lt;obj&gt; = &lt;function&gt;(&lt;keyword_args&gt;)                       <span class="hljs-comment"># E.g. `func(x=0, y=0)`.</span>
&lt;obj&gt; = &lt;function&gt;(&lt;positional_args&gt;, &lt;keyword_args&gt;)    <span class="hljs-comment"># E.g. `func(0, y=0)`.</span>
</code></pre></div>

<div><h2 id="splatoperator"><a href="#splatoperator" name="splatoperator">#</a>Splat Operator</h2><p><strong>Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.</strong></p><pre><code class="python language-python hljs">args, kwargs = (<span class="hljs-number">1</span>, <span class="hljs-number">2</span>), {<span class="hljs-string">'z'</span>: <span class="hljs-number">3</span>}
func(*args, **kwargs)
</code></pre></div>


<div><h4 id="isthesameas">Is the same as:</h4><pre><code class="python language-python hljs">func(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, z=<span class="hljs-number">3</span>)
</code></pre></div>

<div><h3 id="insidefunctiondefinition">Inside Function Definition</h3><p><strong>Splat combines zero or more positional arguments into a tuple, while splatty-splat combines zero or more keyword arguments into a dictionary.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(*a)</span>:</span>
    <span class="hljs-keyword">return</span> sum(a)
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>add(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
<span class="hljs-number">6</span>
</code></pre>
<div><h4 id="allowedcompositionsofargumentsandthewaystheycanbecalled">Allowed compositions of arguments and the ways they can be called:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┓
┃                           │  func(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>)  │ func(<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>) │ func(x=<span class="hljs-number">1</span>, y=<span class="hljs-number">2</span>) ┃
┠───────────────────────────┼──────────────┼──────────────┼────────────────┨
┃ <span class="hljs-title">func</span>(x, *args, **kwargs): │      ✓       │      ✓       │       ✓        ┃
┃ <span class="hljs-title">func</span>(*args, y, **kwargs): │              │      ✓       │       ✓        ┃
┃ <span class="hljs-title">func</span>(*, x, **kwargs):     │              │              │       ✓        ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┛
</code></pre></div>

<div><h3 id="otheruses">Other Uses</h3><pre><code class="python language-python hljs">&lt;list&gt;  = [*&lt;collection&gt; [, ...]]  <span class="hljs-comment"># Or: list(&lt;coll&gt;) [+ ...]</span>
&lt;tuple&gt; = (*&lt;collection&gt;, [...])   <span class="hljs-comment"># Or: tuple(&lt;coll&gt;) [+ ...]</span>
&lt;set&gt;   = {*&lt;collection&gt; [, ...]}  <span class="hljs-comment"># Or: set(&lt;coll&gt;) [| ...]</span>
&lt;dict&gt;  = {**&lt;dict&gt; [, ...]}       <span class="hljs-comment"># Or: &lt;dict&gt; | ...</span>
</code></pre></div>

<pre><code class="python language-python hljs">head, *body, tail = &lt;collection&gt;   <span class="hljs-comment"># Head or tail can be omitted.</span>
</code></pre>
<div><h2 id="inline"><a href="#inline" name="inline">#</a>Inline</h2><div><h3 id="lambda">Lambda</h3><pre><code class="python language-python hljs">&lt;func&gt; = <span class="hljs-keyword">lambda</span>: &lt;return_value&gt;                     <span class="hljs-comment"># A single statement function.</span>
&lt;func&gt; = <span class="hljs-keyword">lambda</span> &lt;arg_1&gt;, &lt;arg_2&gt;: &lt;return_value&gt;    <span class="hljs-comment"># Also allows default arguments.</span>
</code></pre></div></div>


<div><h3 id="comprehensions">Comprehensions</h3><pre><code class="python language-python hljs">&lt;list&gt; = [i+<span class="hljs-number">1</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)]                   <span class="hljs-comment"># Returns `[1, 2, 3, 4, ..., 10]`.</span>
&lt;iter&gt; = (i <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>) <span class="hljs-keyword">if</span> i &gt; <span class="hljs-number">5</span>)            <span class="hljs-comment"># Returns `iter([6, 7, 8, 9])`.</span>
&lt;set&gt;  = {i+<span class="hljs-number">5</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)}                   <span class="hljs-comment"># Returns `{5, 6, 7, 8, ..., 14}`.</span>
&lt;dict&gt; = {i: i*<span class="hljs-number">2</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">1</span>, <span class="hljs-number">10</span>)}             <span class="hljs-comment"># Returns `{1: 2, 2: 4, ..., 9: 18}`.</span>
</code></pre></div>

<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>[l+r <span class="hljs-keyword">for</span> l <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span> <span class="hljs-keyword">for</span> r <span class="hljs-keyword">in</span> <span class="hljs-string">'abc'</span>]             <span class="hljs-comment"># Inner loop is on the right side.</span>
[<span class="hljs-string">'aa'</span>, <span class="hljs-string">'ab'</span>, <span class="hljs-string">'ac'</span>, ..., <span class="hljs-string">'cc'</span>]
</code></pre>
<div><h3 id="mapfilterreduce">Map, Filter, Reduce</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> reduce
</code></pre></div>

<pre><code class="python language-python hljs">&lt;iter&gt; = map(<span class="hljs-keyword">lambda</span> x: x + <span class="hljs-number">1</span>, range(<span class="hljs-number">10</span>))            <span class="hljs-comment"># Returns `iter([1, 2, ..., 10])`.</span>
&lt;iter&gt; = filter(<span class="hljs-keyword">lambda</span> x: x &gt; <span class="hljs-number">5</span>, range(<span class="hljs-number">10</span>))         <span class="hljs-comment"># Returns `iter([6, 7, 8, 9])`.</span>
&lt;obj&gt;  = reduce(<span class="hljs-keyword">lambda</span> out, x: out + x, range(<span class="hljs-number">10</span>))  <span class="hljs-comment"># Returns 45. Accepts 'initial'.</span>
</code></pre>
<div><h3 id="anyall">Any, All</h3><pre><code class="python language-python hljs">&lt;bool&gt; = any(&lt;collection&gt;)                          <span class="hljs-comment"># Is `bool(&lt;el&gt;)` True for any el?</span>
&lt;bool&gt; = all(&lt;collection&gt;)                          <span class="hljs-comment"># True for all? Also True if empty.</span>
</code></pre></div>

<div><h3 id="conditionalexpression">Conditional Expression</h3><pre><code class="python language-python hljs">&lt;obj&gt; = &lt;exp&gt; <span class="hljs-keyword">if</span> &lt;condition&gt; <span class="hljs-keyword">else</span> &lt;exp&gt;             <span class="hljs-comment"># Only one expression is evaluated.</span>
</code></pre></div>

<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>[i <span class="hljs-keyword">if</span> i <span class="hljs-keyword">else</span> <span class="hljs-string">'zero'</span> <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> (<span class="hljs-number">0</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)]      <span class="hljs-comment"># `any([0, '', [], None]) == False`.</span>
[<span class="hljs-string">'zero'</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre>
<div><h3 id="andor">And, Or</h3><pre><code class="python language-python hljs">&lt;obj&gt; = &lt;exp&gt; <span class="hljs-keyword">and</span> &lt;exp&gt; [<span class="hljs-keyword">and</span> ...]                   <span class="hljs-comment"># Returns first false or last obj.</span>
&lt;obj&gt; = &lt;exp&gt; <span class="hljs-keyword">or</span> &lt;exp&gt; [<span class="hljs-keyword">or</span> ...]                     <span class="hljs-comment"># Returns first true or last obj.</span>
</code></pre></div>

<div><h3 id="walrusoperator">Walrus Operator</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>[i <span class="hljs-keyword">for</span> ch <span class="hljs-keyword">in</span> <span class="hljs-string">'0123'</span> <span class="hljs-keyword">if</span> (i := int(ch)) &gt; <span class="hljs-number">0</span>]      <span class="hljs-comment"># Assigns to var in mid-sentence.</span>
[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
</code></pre></div>

<div><h3 id="namedtupleenumdataclass">Named Tuple, Enum, Dataclass</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> namedtuple
Point = namedtuple(<span class="hljs-string">'Point'</span>, <span class="hljs-string">'x y'</span>)                  <span class="hljs-comment"># Creates tuple's subclass.</span>
point = Point(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>)                                 <span class="hljs-comment"># Returns its instance.</span>

<span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum
Direction = Enum(<span class="hljs-string">'Direction'</span>, <span class="hljs-string">'N E S W'</span>)            <span class="hljs-comment"># Creates an enumeration.</span>
direction = Direction.N                             <span class="hljs-comment"># Returns its member.</span>

<span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
Player = make_dataclass(<span class="hljs-string">'Player'</span>, [<span class="hljs-string">'loc'</span>, <span class="hljs-string">'dir'</span>])   <span class="hljs-comment"># Creates a normal class.</span>
player = Player(point, direction)                   <span class="hljs-comment"># Returns its instance.</span>
</code></pre></div>

<div><h2 id="imports"><a href="#imports" name="imports">#</a>Imports</h2><p><strong>Mechanism that makes code in one file available to another file.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> &lt;module&gt;                <span class="hljs-comment"># Imports a built-in or '&lt;module&gt;.py'.</span>
<span class="hljs-keyword">import</span> &lt;package&gt;               <span class="hljs-comment"># Imports a built-in or '&lt;package&gt;/__init__.py'.</span>
<span class="hljs-keyword">import</span> &lt;package&gt;.&lt;module&gt;      <span class="hljs-comment"># Imports a built-in or '&lt;package&gt;/&lt;module&gt;.py'.</span>
</code></pre></div>


<ul>
<li><strong>Package is a collection of modules, but it can also define its own functions, classes, etc.</strong></li>
<li><strong>On a filesystem this corresponds to a directory of Python files with an optional init script.</strong></li>
<li><strong>Running <code class="python hljs"><span class="hljs-string">'import &lt;package&gt;'</span></code> does not automatically provide access to the package's modules unless they are explicitly imported in the <code class="python hljs"><span class="hljs-string">'&lt;package&gt;/__init__.py'</span></code> script.</strong></li>
<li><strong>Directory of the file that is passed to python command serves as a root of local imports.</strong></li>
<li><strong>For relative imports use <code class="python hljs"><span class="hljs-string">'from .[…][&lt;pkg/module&gt;[.…]] import &lt;obj&gt;'</span></code>.</strong></li>
</ul>
<div><h2 id="closure"><a href="#closure" name="closure">#</a>Closure</h2><p><strong>We have/get a closure in Python when a nested function references a value of its enclosing function and then the enclosing function returns its nested function.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_multiplier</span><span class="hljs-params">(a)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(b)</span>:</span>
        <span class="hljs-keyword">return</span> a * b
    <span class="hljs-keyword">return</span> out
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>multiply_by_3 = get_multiplier(<span class="hljs-number">3</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<ul>
<li><strong>Any value that is referenced from within multiple nested functions gets shared.</strong></li>
</ul>
<div><h3 id="partial">Partial</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> partial
&lt;function&gt; = partial(&lt;function&gt; [, &lt;arg_1&gt; [, ...]])
</code></pre></div>

<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">multiply</span><span class="hljs-params">(a, b)</span>:</span>
<span class="hljs-meta">... </span>    <span class="hljs-keyword">return</span> a * b
<span class="hljs-meta">&gt;&gt;&gt; </span>multiply_by_3 = partial(multiply, <span class="hljs-number">3</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>multiply_by_3(<span class="hljs-number">10</span>)
<span class="hljs-number">30</span>
</code></pre>
<ul>
<li><strong>Partial is also useful in cases when a function needs to be passed as an argument because it enables us to set its arguments beforehand (<code class="python hljs"><span class="hljs-string">'collections.defaultdict(&lt;func&gt;)'</span></code>, <code class="python hljs"><span class="hljs-string">'iter(&lt;func&gt;, to_exc)'</span></code> and <code class="python hljs"><span class="hljs-string">'dataclasses.field(default_factory=&lt;func&gt;)'</span></code>).</strong></li>
</ul>
<div><h3 id="nonlocal">Non-Local</h3><p><strong>If variable is being assigned to anywhere in the scope, it is regarded as a local variable, unless it is declared as a 'global' or a 'nonlocal'.</strong></p><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_counter</span><span class="hljs-params">()</span>:</span>
    i = <span class="hljs-number">0</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">()</span>:</span>
        <span class="hljs-keyword">nonlocal</span> i
        i += <span class="hljs-number">1</span>
        <span class="hljs-keyword">return</span> i
    <span class="hljs-keyword">return</span> out
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>counter = get_counter()
<span class="hljs-meta">&gt;&gt;&gt; </span>counter(), counter(), counter()
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div class="pagebreak"></div><div><h2 id="decorator"><a href="#decorator" name="decorator">#</a>Decorator</h2><p><strong>A decorator takes a function, adds some functionality and returns it. It can be any <a href="#callable">callable</a>, but is usually implemented as a function that returns a <a href="#closure">closure</a>.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">@decorator_name</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">function_that_gets_passed_to_decorator</span><span class="hljs-params">()</span>:</span>
    ...
</code></pre></div>


<div><h3 id="debuggerexample">Debugger Example</h3><p><strong>Decorator that prints function's name every time <em>that</em> function is called.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta">    @wraps(func)</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
        print(func.__name__)
        <span class="hljs-keyword">return</span> func(*args, **kwargs)
    <span class="hljs-keyword">return</span> out

<span class="hljs-meta">@debug</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
    <span class="hljs-keyword">return</span> x + y
</code></pre></div>


<ul>
<li><strong>Wraps is a helper decorator that copies the metadata of the passed function (func) to the function it is decorating (out). Without it, <code class="python hljs"><span class="hljs-string">'add.__name__'</span></code> would return <code class="python hljs"><span class="hljs-string">'out'</span></code>.</strong></li>
</ul>
<div><h3 id="cache">Cache</h3><p><strong>Decorator that caches function's return values. All function's arguments must be hashable.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> cache

<span class="hljs-meta">@cache</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">fibonacci</span><span class="hljs-params">(n)</span>:</span>
    <span class="hljs-keyword">return</span> n <span class="hljs-keyword">if</span> n &lt; <span class="hljs-number">2</span> <span class="hljs-keyword">else</span> fibonacci(n<span class="hljs-number">-2</span>) + fibonacci(n<span class="hljs-number">-1</span>)
</code></pre></div>


<ul>
<li><strong>Potential problem with cache is that it can grow indefinitely. To clear stored values run <code class="python hljs"><span class="hljs-string">'&lt;func&gt;.cache_clear()'</span></code>, or use <code class="python hljs"><span class="hljs-string">'@lru_cache(maxsize=&lt;int&gt;)'</span></code> decorator instead.</strong></li>
<li><strong>CPython interpreter limits recursion depth to 3000 by default. To increase the limit run <code class="python hljs"><span class="hljs-string">'sys.setrecursionlimit(&lt;int&gt;)'</span></code>.</strong></li>
</ul>
<div><h3 id="parametrizeddecorator">Parametrized Decorator</h3><p><strong>A decorator that accepts arguments and returns a normal decorator that accepts a function.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> wraps

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">debug</span><span class="hljs-params">(print_result=<span class="hljs-keyword">False</span>)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">decorator</span><span class="hljs-params">(func)</span>:</span>
<span class="hljs-meta">        @wraps(func)</span>
        <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">out</span><span class="hljs-params">(*args, **kwargs)</span>:</span>
            result = func(*args, **kwargs)
            print(func.__name__, result <span class="hljs-keyword">if</span> print_result <span class="hljs-keyword">else</span> <span class="hljs-string">''</span>)
            <span class="hljs-keyword">return</span> result
        <span class="hljs-keyword">return</span> out
    <span class="hljs-keyword">return</span> decorator

<span class="hljs-meta">@debug(print_result=True)</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">add</span><span class="hljs-params">(x, y)</span>:</span>
    <span class="hljs-keyword">return</span> x + y
</code></pre></div>


<ul>
<li><strong>Using only <code class="python hljs"><span class="hljs-string">'@debug'</span></code> to decorate the add() function would not work here, because debug would then receive the add() function as a 'print_result' argument. Decorators can how­ever manually check if the argument they received is a function and act accordingly.</strong></li>
</ul>
<div><h2 id="class"><a href="#class" name="class">#</a>Class</h2><p><strong>A template for creating user-defined objects.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClass</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__str__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> str(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
        class_name = self.__class__.__name__
        <span class="hljs-keyword">return</span> <span class="hljs-string">f'<span class="hljs-subst">{class_name}</span>(<span class="hljs-subst">{self.a!r}</span>)'</span>

<span class="hljs-meta">    @classmethod</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_class_name</span><span class="hljs-params">(cls)</span>:</span>
        <span class="hljs-keyword">return</span> cls.__name__
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>obj = MyClass(<span class="hljs-number">1</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>obj.a, str(obj), repr(obj)
(<span class="hljs-number">1</span>, <span class="hljs-string">'1'</span>, <span class="hljs-string">'MyClass(1)'</span>)
</code></pre>
<ul>
<li><strong>Methods whose names start and end with two underscores are called special methods. They are executed when object is passed to a built-in function or used as an operand, for&nbsp;example, <code class="python hljs"><span class="hljs-string">'print(a)'</span></code> calls <code class="python hljs"><span class="hljs-string">'a.__str__()'</span></code> and <code class="python hljs"><span class="hljs-string">'a + b'</span></code> calls <code class="python hljs"><span class="hljs-string">'a.__add__(b)'</span></code>.</strong></li>
<li><strong>Methods decorated with <code class="python hljs"><span class="hljs-string">'@staticmethod'</span></code> receive neither 'self' nor 'cls' argument.</strong></li>
<li><strong>Return value of str() special method should be readable and of repr() unambiguous. If only repr() is defined, it will also be used for str().</strong></li>
</ul>
<div><h4 id="expressionsthatcallobjectsstrspecialmethod">Expressions that call object's str() special method:</h4><pre><code class="python language-python hljs">print(obj)
<span class="hljs-string">f'<span class="hljs-subst">{obj}</span>'</span>
logging.warning(obj)
csv.writer(&lt;file&gt;).writerow([obj])
</code></pre></div>

<div><h4 id="expressionsthatcallobjectsreprspecialmethod">Expressions that call object's repr() special method:</h4><pre><code class="python language-python hljs">print/str/repr([obj])
print/str/repr({obj: obj})
<span class="hljs-string">f'<span class="hljs-subst">{obj!r}</span>'</span>
Z = make_dataclass(<span class="hljs-string">'Z'</span>, [<span class="hljs-string">'a'</span>]); print/str/repr(Z(obj))
</code></pre></div>

<div><h3 id="subclass">Subclass</h3><ul>
<li><strong>Inheritance is a mechanism that enables a class to extend some other class (that is, subclass to extend its parent), and by doing so inherit all of its methods and attributes.</strong></li>
<li><strong>Subclass can then add its own methods and attributes or override inherited ones by reusing their names.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name)</span>:</span>
        self.name = name
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> <span class="hljs-string">f'Person(<span class="hljs-subst">{self.name!r}</span>)'</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__lt__</span><span class="hljs-params">(self, other)</span>:</span>
        <span class="hljs-keyword">return</span> self.name &lt; other.name

<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Employee</span><span class="hljs-params">(Person)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, name, staff_num)</span>:</span>
        super().__init__(name)
        self.staff_num = staff_num
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__repr__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> <span class="hljs-string">f'Employee(<span class="hljs-subst">{self.name!r}</span>, <span class="hljs-subst">{self.staff_num}</span>)'</span>
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>people = {Person(<span class="hljs-string">'Ann'</span>), Employee(<span class="hljs-string">'Bob'</span>, <span class="hljs-number">0</span>)}
<span class="hljs-meta">&gt;&gt;&gt; </span>sorted(people)
[Person(<span class="hljs-string">'Ann'</span>), Employee(<span class="hljs-string">'Bob'</span>, <span class="hljs-number">0</span>)]
</code></pre>
<div><h3 id="typeannotations">Type Annotations</h3><ul>
<li><strong>They add type hints to variables, arguments and functions (<code class="python hljs"><span class="hljs-string">'def f() -&gt; &lt;type&gt;:'</span></code>).</strong></li>
<li><strong>Hints are used by type checkers like <a href="https://pypi.org/project/mypy/">mypy</a>, data validation libraries such as <a href="https://pypi.org/project/pydantic/">Pydantic</a> and lately also by <a href="https://pypi.org/project/Cython/">Cython</a> compiler. However, they are not enforced by CPython interpreter.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> abc

&lt;name&gt;: &lt;type&gt; [| ...] [= &lt;obj&gt;]
&lt;name&gt;: list/set/abc.Iterable/abc.Sequence[&lt;type&gt;] [= &lt;obj&gt;]
&lt;name&gt;: tuple/dict[&lt;type&gt;, ...] [= &lt;obj&gt;]
</code></pre></div>


<div><h3 id="dataclass">Dataclass</h3><p><strong>Decorator that uses class variables to generate init(), repr() and eq() special methods.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> dataclass, field, make_dataclass

<span class="hljs-meta">@dataclass(order=False, frozen=False)</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> &lt;<span class="hljs-title">class_name</span>&gt;:</span>
    &lt;attr_name&gt;: &lt;type&gt;
    &lt;attr_name&gt;: &lt;type&gt; = &lt;default_value&gt;
    &lt;attr_name&gt;: list/dict/set = field(default_factory=list/dict/set)
</code></pre></div>


<ul>
<li><strong>Objects can be made <a href="#sortable">sortable</a> with <code class="python hljs"><span class="hljs-string">'order=True'</span></code> and immutable with <code class="python hljs"><span class="hljs-string">'frozen=True'</span></code>.</strong></li>
<li><strong>For object to be <a href="#hashable">hashable</a>, all attributes must be hashable and 'frozen' must be True.</strong></li>
<li><strong>Function field() is needed because <code class="python hljs"><span class="hljs-string">'&lt;attr_name&gt;: list = []'</span></code> would make a list that is shared among all instances. Its 'default_factory' argument accepts any <a href="#callable">callable</a> object.</strong></li>
<li><strong>For attributes of arbitrary type use <code class="python hljs"><span class="hljs-string">'typing.Any'</span></code>.</strong></li>
</ul>
<pre><code class="python language-python hljs">P = make_dataclass(<span class="hljs-string">'P'</span>, [<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float), (<span class="hljs-string">'y'</span>, float)])
P = make_dataclass(<span class="hljs-string">'P'</span>, [(<span class="hljs-string">'x'</span>, float, <span class="hljs-number">0</span>), (<span class="hljs-string">'y'</span>, float, <span class="hljs-number">0</span>)])
</code></pre>
<div><h3 id="property">Property</h3><p><strong>Pythonic way of implementing getters and setters.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Person</span>:</span>
<span class="hljs-meta">    @property</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> <span class="hljs-string">' '</span>.join(self._name)

<span class="hljs-meta">    @name.setter</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">name</span><span class="hljs-params">(self, value)</span>:</span>
        self._name = value.split()
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>person = Person()
<span class="hljs-meta">&gt;&gt;&gt; </span>person.name = <span class="hljs-string">'\t Guido  van Rossum \n'</span>
<span class="hljs-meta">&gt;&gt;&gt; </span>person.name
<span class="hljs-string">'Guido van Rossum'</span>
</code></pre>
<div><h3 id="slots">Slots</h3><p><strong>Mechanism that restricts objects to attributes listed in 'slots'.</strong></p><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyClassWithSlots</span>:</span>
    __slots__ = [<span class="hljs-string">'a'</span>]
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
        self.a = <span class="hljs-number">1</span>
</code></pre></div>


<div><h3 id="copy">Copy</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> copy <span class="hljs-keyword">import</span> copy, deepcopy
&lt;object&gt; = copy/deepcopy(&lt;object&gt;)
</code></pre></div>

<div><h2 id="ducktypes"><a href="#ducktypes" name="ducktypes">#</a>Duck Types</h2><p><strong>A duck type is an implicit type that prescribes a set of special methods. Any object that has those methods defined is considered a member of that duck type.</strong></p><div><h3 id="comparable">Comparable</h3><ul>
<li><strong>If eq() method is not overridden, it returns <code class="python hljs"><span class="hljs-string">'id(self) == id(other)'</span></code>, which is the same as <code class="python hljs"><span class="hljs-string">'self is other'</span></code>. That means all user-defined objects compare not equal by default (because id() returns object's memory address that is guaranteed to be unique).</strong></li>
<li><strong>Only the left side object has eq() method called, unless it returns NotImplemented, in which case the right object is consulted. Result is False if both return NotImplemented.</strong></li>
<li><strong>Ne() automatically works on any object that has eq() defined.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyComparable</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
        <span class="hljs-keyword">if</span> isinstance(other, type(self)):
            <span class="hljs-keyword">return</span> self.a == other.a
        <span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div></div>




<div><h3 id="hashable">Hashable</h3><ul>
<li><strong>Hashable object needs both hash() and eq() methods and its hash value must not change.</strong></li>
<li><strong>Hashable objects that compare equal must have the same hash value, meaning default hash() that returns <code class="python hljs"><span class="hljs-string">'id(self)'</span></code> will not do. That is why Python automatically makes classes unhashable if you only implement eq().</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyHashable</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self._a = a
<span class="hljs-meta">    @property</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">a</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> self._a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
        <span class="hljs-keyword">if</span> isinstance(other, type(self)):
            <span class="hljs-keyword">return</span> self.a == other.a
        <span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__hash__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> hash(self.a)
</code></pre></div>


<div><h3 id="sortable">Sortable</h3><ul>
<li><strong>With 'total_ordering' decorator, you only need to provide eq() and one of lt(), gt(), le() or ge() special methods (used by &lt;, &gt;, &lt;=, &gt;=) and the rest will be automatically generated.</strong></li>
<li><strong>Functions sorted() and min() only require lt() method, while max() only requires gt(). However, it is best to define them all so that confusion doesn't arise in other contexts.</strong></li>
<li><strong>When two lists, strings or dataclasses are compared, their values get compared in order until a pair of unequal values is found. The comparison of this two values is then re­turned. The shorter sequence is considered smaller in case of all values being equal.</strong></li>
<li><strong>To sort collection of strings in proper alphabetical order pass <code class="python hljs"><span class="hljs-string">'key=locale.strxfrm'</span></code> to sorted() after running <code class="python hljs"><span class="hljs-string">'locale.setlocale(locale.LC_COLLATE, "en_US.UTF-8")'</span></code>.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> total_ordering

<span class="hljs-meta">@total_ordering</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MySortable</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__eq__</span><span class="hljs-params">(self, other)</span>:</span>
        <span class="hljs-keyword">if</span> isinstance(other, type(self)):
            <span class="hljs-keyword">return</span> self.a == other.a
        <span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__lt__</span><span class="hljs-params">(self, other)</span>:</span>
        <span class="hljs-keyword">if</span> isinstance(other, type(self)):
            <span class="hljs-keyword">return</span> self.a &lt; other.a
        <span class="hljs-keyword">return</span> <span class="hljs-built_in">NotImplemented</span>
</code></pre></div>


<div><h3 id="iterator-1">Iterator</h3><ul>
<li><strong>Any object that has methods next() and iter() is an iterator.</strong></li>
<li><strong>Next() should return next item or raise StopIteration exception.</strong></li>
<li><strong>Iter() should return unmodified iterator, i.e. the 'self' argument.</strong></li>
<li><strong>Any object that has iter() method can be used in a for loop.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Counter</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
        self.i = <span class="hljs-number">0</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__next__</span><span class="hljs-params">(self)</span>:</span>
        self.i += <span class="hljs-number">1</span>
        <span class="hljs-keyword">return</span> self.i
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> self
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>counter = Counter()
<span class="hljs-meta">&gt;&gt;&gt; </span>next(counter), next(counter), next(counter)
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div><h4 id="pythonhasmanydifferentiteratorobjects">Python has many different iterator objects:</h4><ul>
<li><strong>Sequence iterators returned by the <a href="#iterator">iter()</a> function, such as list_iterator, etc.</strong></li>
<li><strong>Objects returned by the <a href="#itertools">itertools</a> module, such as count, repeat and cycle.</strong></li>
<li><strong>Generators returned by the <a href="#generator">generator functions</a> and <a href="#comprehensions">generator expressions</a>.</strong></li>
<li><strong>File objects returned by the <a href="#open">open()</a> function, etc.</strong></li>
</ul><div><h3 id="callable">Callable</h3><ul>
<li><strong>All functions and classes have a call() method that is executed when they are called.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'callable(&lt;obj&gt;)'</span></code> or <code class="python hljs"><span class="hljs-string">'isinstance(&lt;obj&gt;, collections.abc.Callable)'</span></code> to check if object is callable. You can also call the object and check if it raised TypeError.</strong></li>
<li><strong>When this cheatsheet uses <code class="python hljs"><span class="hljs-string">'&lt;function&gt;'</span></code> as an argument, it means <code class="python hljs"><span class="hljs-string">'&lt;callable&gt;'</span></code>.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Counter</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self)</span>:</span>
        self.i = <span class="hljs-number">0</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__call__</span><span class="hljs-params">(self)</span>:</span>
        self.i += <span class="hljs-number">1</span>
        <span class="hljs-keyword">return</span> self.i
</code></pre></div></div>




<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>counter = Counter()
<span class="hljs-meta">&gt;&gt;&gt; </span>counter(), counter(), counter()
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<div><h3 id="contextmanager">Context Manager</h3><ul>
<li><strong>With statements only work on objects that have enter() and exit() special methods.</strong></li>
<li><strong>Enter() should lock the resources and optionally return an object (file, lock, etc.).</strong></li>
<li><strong>Exit() should release the resources (for example close a file, release a lock, etc.).</strong></li>
<li><strong>Any exception that happens inside the with block is passed to the exit() method.</strong></li>
<li><strong>The exit() method can suppress the exception by returning a true value.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyOpen</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, filename)</span>:</span>
        self.filename = filename
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__enter__</span><span class="hljs-params">(self)</span>:</span>
        self.file = open(self.filename)
        <span class="hljs-keyword">return</span> self.file
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__exit__</span><span class="hljs-params">(self, exc_type, exception, traceback)</span>:</span>
        self.file.close()
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">with</span> open(<span class="hljs-string">'test.txt'</span>, <span class="hljs-string">'w'</span>) <span class="hljs-keyword">as</span> file:
<span class="hljs-meta">... </span>    file.write(<span class="hljs-string">'Hello World!'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">with</span> MyOpen(<span class="hljs-string">'test.txt'</span>) <span class="hljs-keyword">as</span> file:
<span class="hljs-meta">... </span>    print(file.read())
Hello World!
</code></pre>
<div><h2 id="iterableducktypes"><a href="#iterableducktypes" name="iterableducktypes">#</a>Iterable Duck Types</h2><div><h3 id="iterable">Iterable</h3><ul>
<li><strong>Only required method is iter(). It should return an iterator of object's items.</strong></li>
<li><strong>Contains() automatically works on any object that has iter() defined.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyIterable</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> iter(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__contains__</span><span class="hljs-params">(self, el)</span>:</span>
        <span class="hljs-keyword">return</span> el <span class="hljs-keyword">in</span> self.a
</code></pre></div></div>



<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>obj = MyIterable([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>])
<span class="hljs-meta">&gt;&gt;&gt; </span>[el <span class="hljs-keyword">for</span> el <span class="hljs-keyword">in</span> obj]
[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>]
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-number">1</span> <span class="hljs-keyword">in</span> obj
<span class="hljs-keyword">True</span>
</code></pre>
<div><h3 id="collection">Collection</h3><ul>
<li><strong>Only required methods are iter() and len(). Len() should return the number of items.</strong></li>
<li><strong>This cheatsheet actually means <code class="python hljs"><span class="hljs-string">'&lt;iterable&gt;'</span></code> when it uses the <code class="python hljs"><span class="hljs-string">'&lt;collection&gt;'</span></code>.</strong></li>
<li><strong>I chose not to use the name 'iterable' because it sounds scarier and more vague than 'collection'. The main drawback of this decision is that the reader could think a certain function doesn't accept iterators when it does, since iterators are the only built-in objects that are iterable but are not collections.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyCollection</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> iter(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__contains__</span><span class="hljs-params">(self, el)</span>:</span>
        <span class="hljs-keyword">return</span> el <span class="hljs-keyword">in</span> self.a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__len__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> len(self.a)
</code></pre></div>


<div><h3 id="sequence">Sequence</h3><ul>
<li><strong>Only required methods are getitem() and len(). Getitem() should return the item at the passed index or raise IndexError (it may also support negative indices and/or slices).</strong></li>
<li><strong>Iter() and contains() automatically work on any object that has getitem() defined.</strong></li>
<li><strong>Reversed() automatically works on any object that has getitem() and len() defined. It returns reversed iterator of object's items.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MySequence</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__iter__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> iter(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__contains__</span><span class="hljs-params">(self, el)</span>:</span>
        <span class="hljs-keyword">return</span> el <span class="hljs-keyword">in</span> self.a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__len__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> len(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__getitem__</span><span class="hljs-params">(self, i)</span>:</span>
        <span class="hljs-keyword">return</span> self.a[i]
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__reversed__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> reversed(self.a)
</code></pre></div>


<div><h4 id="discrepanciesbetweenglossarydefinitionsandabstractbaseclasses">Discrepancies between glossary definitions and abstract base classes:</h4><ul>
<li><strong>Python's glossary defines iterable as any object with special methods iter() and/or getitem() and sequence as any object with getitem() and len(). It doesn't define collection.</strong></li>
<li><strong>Passing ABC Iterable to isinstance() or issubclass() only checks whether object/class has special method iter(), while ABC Collection checks for iter(), contains() and len().</strong></li>
</ul></div>




<div><h3 id="abcsequence">ABC Sequence</h3><ul>
<li><strong>It's a richer interface than the basic sequence that also requires just getitem() and len().</strong></li>
<li><strong>Extending it generates iter(), contains(), reversed(), index() and count() special methods.</strong></li>
<li><strong>Unlike <code class="python hljs"><span class="hljs-string">'abc.Iterable'</span></code> and <code class="python hljs"><span class="hljs-string">'abc.Collection'</span></code>, it is not a duck type. That is why <code class="python hljs"><span class="hljs-string">'issubclass(MySequence, abc.Sequence)'</span></code> would return False even if MySequence had all the methods defined. It however recognizes list, tuple, range, str, bytes, bytearray, array, memoryview and deque, since they are registered as Sequence's virtual subclasses.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> abc

<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyAbcSequence</span><span class="hljs-params">(abc.Sequence)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, a)</span>:</span>
        self.a = a
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__len__</span><span class="hljs-params">(self)</span>:</span>
        <span class="hljs-keyword">return</span> len(self.a)
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__getitem__</span><span class="hljs-params">(self, i)</span>:</span>
        <span class="hljs-keyword">return</span> self.a[i]
</code></pre></div><div><h4 id="tableofrequiredandautomaticallyavailablespecialmethods">Table of required and automatically available special methods:</h4><pre><code class="text language-text">┏━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━━━┓
┃            │  Iterable  │ Collection │  Sequence  │ abc.Sequence ┃
┠────────────┼────────────┼────────────┼────────────┼──────────────┨
┃ iter()     │     !      │     !      │     ✓      │      ✓       ┃
┃ contains() │     ✓      │     ✓      │     ✓      │      ✓       ┃
┃ len()      │            │     !      │     !      │      !       ┃
┃ getitem()  │            │            │     !      │      !       ┃
┃ reversed() │            │            │     ✓      │      ✓       ┃
┃ index()    │            │            │            │      ✓       ┃
┃ count()    │            │            │            │      ✓       ┃
┗━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━━━┛
</code></pre></div>

<ul>
<li><strong>Method iter() is required for <code class="python hljs"><span class="hljs-string">'isinstance(&lt;obj&gt;, abc.Iterable)'</span></code> to return True, however any object with getitem() will work with any code expecting an iterable.</strong></li>
<li><strong>MutableSequence, Set, MutableSet, Mapping and MutableMapping ABCs are also extendable. Use <code class="python hljs"><span class="hljs-string">'&lt;abc&gt;.__abstractmethods__'</span></code> to get names of required methods.</strong></li>
</ul>
<div><h2 id="enum"><a href="#enum" name="enum">#</a>Enum</h2><p><strong>Class of named constants called members.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum, auto
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> &lt;<span class="hljs-title">enum_name</span>&gt;<span class="hljs-params">(Enum)</span>:</span>
    &lt;member_name&gt; = auto()              <span class="hljs-comment"># An increment of last numeric value or 1.</span>
    &lt;member_name&gt; = &lt;value&gt;             <span class="hljs-comment"># Values don't have to be hashable/unique.</span>
    &lt;member_name&gt; = &lt;el_1&gt;, &lt;el_2&gt;      <span class="hljs-comment"># Value can be a collection, e.g. tuple.</span>
</code></pre>
<ul>
<li><strong>Methods receive the member they were called on as the 'self' argument.</strong></li>
<li><strong>Accessing a member named after a reserved keyword causes SyntaxError.</strong></li>
</ul>
<pre><code class="python language-python hljs">&lt;member&gt; = &lt;enum&gt;.&lt;member_name&gt;         <span class="hljs-comment"># Accesses a member via enum's attribute.</span>
&lt;member&gt; = &lt;enum&gt;[<span class="hljs-string">'&lt;member_name&gt;'</span>]      <span class="hljs-comment"># Returns the member or raises KeyError.</span>
&lt;member&gt; = &lt;enum&gt;(&lt;value&gt;)              <span class="hljs-comment"># Returns the member or raises ValueError.</span>
&lt;str&gt;    = &lt;member&gt;.name                <span class="hljs-comment"># Returns the member's name as a string.</span>
&lt;obj&gt;    = &lt;member&gt;.value               <span class="hljs-comment"># Value can't be a user-defined function.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt;   = list(&lt;enum&gt;)                 <span class="hljs-comment"># Returns a list of the enum's members.</span>
&lt;list&gt;   = &lt;enum&gt;._member_names_        <span class="hljs-comment"># Returns a list of the member names.</span>
&lt;list&gt;   = [m.value <span class="hljs-keyword">for</span> m <span class="hljs-keyword">in</span> &lt;enum&gt;]    <span class="hljs-comment"># Returns a list of the member values.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;enum&gt;   = type(&lt;member&gt;)               <span class="hljs-comment"># Returns an enum. Also &lt;memb&gt;.__class__.</span>
&lt;iter&gt;   = itertools.cycle(&lt;enum&gt;)      <span class="hljs-comment"># Returns an endless iterator of members.</span>
&lt;member&gt; = random.choice(list(&lt;enum&gt;))  <span class="hljs-comment"># Randomly selects one of the members.</span>
</code></pre>
<div><h3 id="inline-1">Inline</h3><pre><code class="python language-python hljs">Cutlery = Enum(<span class="hljs-string">'Cutlery'</span>, <span class="hljs-string">'FORK KNIFE SPOON'</span>)
Cutlery = Enum(<span class="hljs-string">'Cutlery'</span>, [<span class="hljs-string">'FORK'</span>, <span class="hljs-string">'KNIFE'</span>, <span class="hljs-string">'SPOON'</span>])
Cutlery = Enum(<span class="hljs-string">'Cutlery'</span>, {<span class="hljs-string">'FORK'</span>: <span class="hljs-number">1</span>, <span class="hljs-string">'KNIFE'</span>: <span class="hljs-number">2</span>, <span class="hljs-string">'SPOON'</span>: <span class="hljs-number">3</span>})
</code></pre></div>

<div><h4 id="userdefinedfunctionscannotbevaluessotheymustbewrapped">User-defined functions cannot be values, so they must be wrapped:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> functools <span class="hljs-keyword">import</span> partial
LogicOp = Enum(<span class="hljs-string">'LogicOp'</span>, {<span class="hljs-string">'AND'</span>: partial(<span class="hljs-keyword">lambda</span> l, r: l <span class="hljs-keyword">and</span> r),
                           <span class="hljs-string">'OR'</span>:  partial(<span class="hljs-keyword">lambda</span> l, r: l <span class="hljs-keyword">or</span> r)})
</code></pre></div>

<div><h2 id="exceptions"><a href="#exceptions" name="exceptions">#</a>Exceptions</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">try</span>:
    &lt;code&gt;
<span class="hljs-keyword">except</span> &lt;exception&gt;:
    &lt;code&gt;
</code></pre></div>

<div><h3 id="complexexample">Complex Example</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">try</span>:
    &lt;code_1&gt;
<span class="hljs-keyword">except</span> &lt;exception_a&gt;:
    &lt;code_2_a&gt;
<span class="hljs-keyword">except</span> &lt;exception_b&gt;:
    &lt;code_2_b&gt;
<span class="hljs-keyword">else</span>:
    &lt;code_2_c&gt;
<span class="hljs-keyword">finally</span>:
    &lt;code_3&gt;
</code></pre></div>

<ul>
<li><strong>Code inside the <code class="python hljs"><span class="hljs-string">'else'</span></code> block will only be executed if <code class="python hljs"><span class="hljs-string">'try'</span></code> block had no exceptions.</strong></li>
<li><strong>Code inside the <code class="python hljs"><span class="hljs-string">'finally'</span></code> block will always be executed (unless a signal is received).</strong></li>
<li><strong>All variables that are initialized in executed blocks are also visible in all subsequent blocks, as well as outside the try statement (only the function block delimits scope).</strong></li>
<li><strong>To catch signals use <code class="python hljs"><span class="hljs-string">'signal.signal(signal_number, handler_function)'</span></code>.</strong></li>
</ul>
<div><h3 id="catchingexceptions">Catching Exceptions</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">except</span> &lt;exception&gt;: ...
<span class="hljs-keyword">except</span> &lt;exception&gt; <span class="hljs-keyword">as</span> &lt;name&gt;: ...
<span class="hljs-keyword">except</span> (&lt;exception&gt;, [...]): ...
<span class="hljs-keyword">except</span> (&lt;exception&gt;, [...]) <span class="hljs-keyword">as</span> &lt;name&gt;: ...
</code></pre></div>

<ul>
<li><strong>It also catches subclasses, e.g. <code class="python hljs"><span class="hljs-string">'ArithmeticError'</span></code> is caught by <code class="python hljs"><span class="hljs-string">'except Exception:'</span></code>.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'traceback.print_exc()'</span></code> to print the full error message to standard error stream.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'print(&lt;name&gt;)'</span></code> to print just the cause of the exception (that is, its arguments).</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'logging.exception(&lt;str&gt;)'</span></code> to log the passed message, followed by the full error message of the caught exception. For details about setting up the logger see <a href="#logging">Logging</a>.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'sys.exc_info()'</span></code> to get exception type, object, and traceback of caught exception.</strong></li>
</ul>
<div><h3 id="raisingexceptions">Raising Exceptions</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">raise</span> &lt;exception&gt;
<span class="hljs-keyword">raise</span> &lt;exception&gt;()
<span class="hljs-keyword">raise</span> &lt;exception&gt;(&lt;obj&gt; [, ...])
</code></pre></div>

<div><h4 id="reraisingcaughtexception">Re-raising caught exception:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">except</span> &lt;exception&gt; [<span class="hljs-keyword">as</span> &lt;name&gt;]:
    ...
    <span class="hljs-keyword">raise</span>
</code></pre></div>

<div><h3 id="exceptionobject">Exception Object</h3><pre><code class="python language-python hljs">arguments = &lt;name&gt;.args
exc_type  = &lt;name&gt;.__class__
filename  = &lt;name&gt;.__traceback__.tb_frame.f_code.co_filename
func_name = &lt;name&gt;.__traceback__.tb_frame.f_code.co_name
line_str  = linecache.getline(filename, &lt;name&gt;.__traceback__.tb_lineno)
trace_str = <span class="hljs-string">''</span>.join(traceback.format_tb(&lt;name&gt;.__traceback__))
error_msg = <span class="hljs-string">''</span>.join(traceback.format_exception(*sys.exc_info()))
</code></pre></div>

<div><h3 id="builtinexceptions">Built-in Exceptions</h3><pre><code class="text language-text">BaseException
 ├── SystemExit                   <span class="hljs-comment"># Raised by the sys.exit() function (see #Exit for details).</span>
 ├── KeyboardInterrupt            <span class="hljs-comment"># Raised when the user hits the interrupt key (control-c).</span>
 └── Exception                    <span class="hljs-comment"># User-defined exceptions should be derived from this class.</span>
      ├── ArithmeticError         <span class="hljs-comment"># Base class for arithmetic errors such as ZeroDivisionError.</span>
      ├── AssertionError          <span class="hljs-comment"># Raised by `assert &lt;exp&gt;` if expression returns false value.</span>
      ├── AttributeError          <span class="hljs-comment"># Raised when object doesn't have requested attribute/method.</span>
      ├── EOFError                <span class="hljs-comment"># Is raised by input() when it hits an end-of-file condition.</span>
      ├── LookupError             <span class="hljs-comment"># Base class for errors when a collection can't find an item.</span>
      │    ├── IndexError         <span class="hljs-comment"># Raised when index of a sequence (list/str) is out of range.</span>
      │    └── KeyError           <span class="hljs-comment"># Raised when a dictionary key or a set element is missing.</span>
      ├── MemoryError             <span class="hljs-comment"># Out of memory. May be too late to start deleting variables.</span>
      ├── NameError               <span class="hljs-comment"># Raised when nonexistent name (variable/func/class) is used.</span>
      │    └── UnboundLocalError  <span class="hljs-comment"># Raised when local name is used before it's being defined.</span>
      ├── OSError                 <span class="hljs-comment"># Errors such as FileExistsError, TimeoutError (see #Open).</span>
      │    └── ConnectionError    <span class="hljs-comment"># Errors such as BrokenPipeError and ConnectionAbortedError.</span>
      ├── RuntimeError            <span class="hljs-comment"># Is raised by errors that don't fit into other categories.</span>
      │    ├── NotImplementedEr…  <span class="hljs-comment"># Can be raised by abstract methods or by unfinished code.</span>
      │    └── RecursionError     <span class="hljs-comment"># Raised if max recursion depth is exceeded (3k by default).</span>
      ├── StopIteration           <span class="hljs-comment"># Raised when exhausted (empty) iterator is passed to next().</span>
      ├── TypeError               <span class="hljs-comment"># When an argument of the wrong type is passed to function.</span>
      └── ValueError              <span class="hljs-comment"># When argument has the right type but inappropriate value.</span>
</code></pre></div>

<div><h4 id="collectionsandtheirexceptions">Collections and their exceptions:</h4><pre><code class="text language-text">┏━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┓
┃           │    List    │    Set     │    Dict    ┃
┠───────────┼────────────┼────────────┼────────────┨
┃ getitem() │ IndexError │            │  KeyError  ┃
┃ pop()     │ IndexError │  KeyError  │  KeyError  ┃
┃ remove()  │ ValueError │  KeyError  │            ┃
┃ index()   │ ValueError │            │            ┃
┗━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┛
</code></pre></div>

<div><h4 id="usefulbuiltinexceptions">Useful built-in exceptions:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">raise</span> TypeError(<span class="hljs-string">'Passed argument is of the wrong type!'</span>)
<span class="hljs-keyword">raise</span> ValueError(<span class="hljs-string">'Argument has the right type but its value is off!'</span>)
<span class="hljs-keyword">raise</span> RuntimeError(<span class="hljs-string">'I am too lazy to define my own exception!'</span>)
</code></pre></div>

<div><h3 id="userdefinedexceptions">User-defined Exceptions</h3><pre><code class="python language-python hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyError</span><span class="hljs-params">(Exception)</span>:</span> <span class="hljs-keyword">pass</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyInputError</span><span class="hljs-params">(MyError)</span>:</span> <span class="hljs-keyword">pass</span>
</code></pre></div>

<div><h2 id="exit"><a href="#exit" name="exit">#</a>Exit</h2><p><strong>Exits the interpreter by raising SystemExit exception.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> sys
sys.exit()                     <span class="hljs-comment"># Exits with exit code 0 (success).</span>
sys.exit(&lt;int&gt;)                <span class="hljs-comment"># Exits with the passed exit code.</span>
sys.exit(&lt;obj&gt;)                <span class="hljs-comment"># Prints to stderr and exits with 1.</span>
</code></pre></div>


<div><h2 id="print"><a href="#print" name="print">#</a>Print</h2><pre><code class="python language-python hljs">print(&lt;el_1&gt;, ..., sep=<span class="hljs-string">' '</span>, end=<span class="hljs-string">'\n'</span>, file=sys.stdout, flush=<span class="hljs-keyword">False</span>)
</code></pre></div>

<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'file=sys.stderr'</span></code> or <code class="python hljs"><span class="hljs-string">'sys.stderr.write(&lt;str&gt;)'</span></code> for messages about errors.</strong></li>
<li><strong>Stdout and stderr streams hold output in a buffer until they receive a string containing '\n' or '\r', buffer reaches 4096 characters, <code class="python hljs"><span class="hljs-string">'flush=True'</span></code> is used, or program exits.</strong></li>
</ul>
<div><h3 id="prettyprint">Pretty Print</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> pprint <span class="hljs-keyword">import</span> pprint
pprint(&lt;collection&gt;, width=<span class="hljs-number">80</span>, depth=<span class="hljs-keyword">None</span>, compact=<span class="hljs-keyword">False</span>, sort_dicts=<span class="hljs-keyword">True</span>)
</code></pre></div>

<ul>
<li><strong>Each item is printed on its own line if collection exceeds 'width' characters.</strong></li>
<li><strong>Nested collections that are 'depth' levels deep get printed as '…'.</strong></li>
</ul>
<div><h2 id="input"><a href="#input" name="input">#</a>Input</h2><pre><code class="python language-python hljs">&lt;str&gt; = input()
</code></pre></div>

<ul>
<li><strong>Reads a line from the user input or pipe if present (trailing newline gets stripped).</strong></li>
<li><strong>If argument is passed, it gets printed to the standard output before input is read.</strong></li>
<li><strong>EOFError is raised if user hits EOF (ctrl-d/ctrl-z⏎) or if stream is already exhausted.</strong></li>
</ul>
<div><h2 id="commandlinearguments"><a href="#commandlinearguments" name="commandlinearguments">#</a>Command Line Arguments</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> sys
scripts_path = sys.argv[<span class="hljs-number">0</span>]
arguments    = sys.argv[<span class="hljs-number">1</span>:]
</code></pre></div>

<div><h3 id="argumentparser">Argument Parser</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> argparse <span class="hljs-keyword">import</span> ArgumentParser
p = ArgumentParser(description=&lt;str&gt;)                             <span class="hljs-comment"># Returns a parser object.</span>
p.add_argument(<span class="hljs-string">'-&lt;short_name&gt;'</span>, <span class="hljs-string">'--&lt;name&gt;'</span>, action=<span class="hljs-string">'store_true'</span>)  <span class="hljs-comment"># Flag (defaults to False).</span>
p.add_argument(<span class="hljs-string">'-&lt;short_name&gt;'</span>, <span class="hljs-string">'--&lt;name&gt;'</span>, type=&lt;type&gt;)          <span class="hljs-comment"># Option (defaults to None).</span>
p.add_argument(<span class="hljs-string">'&lt;name&gt;'</span>, type=&lt;type&gt;, nargs=<span class="hljs-number">1</span>)                    <span class="hljs-comment"># Mandatory first argument.</span>
p.add_argument(<span class="hljs-string">'&lt;name&gt;'</span>, type=&lt;type&gt;, nargs=<span class="hljs-string">'+'</span>)                  <span class="hljs-comment"># Mandatory remaining args.</span>
p.add_argument(<span class="hljs-string">'&lt;name&gt;'</span>, type=&lt;type&gt;, nargs=<span class="hljs-string">'?'</span>)                  <span class="hljs-comment"># Optional argument. Also *.</span>
args  = p.parse_args()                                            <span class="hljs-comment"># Exits on parsing error.</span>
&lt;obj&gt; = args.&lt;name&gt;                                               <span class="hljs-comment"># Returns `&lt;type&gt;(&lt;arg&gt;)`.</span>
</code></pre></div>

<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'help=&lt;str&gt;'</span></code> to set argument description that will be displayed in help message.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'default=&lt;obj&gt;'</span></code> to override None as option's or optional argument's default value.</strong></li>
</ul>
<div><h2 id="open"><a href="#open" name="open">#</a>Open</h2><p><strong>Opens a file and returns the corresponding file object.</strong></p><pre><code class="python language-python hljs">&lt;file&gt; = open(&lt;path&gt;, mode=<span class="hljs-string">'r'</span>, encoding=<span class="hljs-keyword">None</span>, newline=<span class="hljs-keyword">None</span>)
</code></pre></div>


<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'encoding=None'</span></code> means that the default encoding is used, which is platform dependent. Best practice is to use <code class="python hljs"><span class="hljs-string">'encoding="utf-8"'</span></code> until it becomes the default (Python 3.15).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'newline=None'</span></code> means all different end of line combinations are converted to '\n' on read, while on write all '\n' characters are converted to system's default line separator.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'newline=""'</span></code> means no conversions take place, but input is still broken into chunks by readline() on every '\n', '\r' and '\r\n'. Passing <code class="python hljs"><span class="hljs-string">'newline="\n"'</span></code> breaks input only on '\n'.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'newline="\r\n"'</span></code> converts every '\n' to '\r\n' on write and breaks input only on '\r\n'.</strong></li>
</ul>
<div><h3 id="modes">Modes</h3><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'r'</span></code>  - Read text from the file (the default option).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'w'</span></code>  - Write to the file. Deletes existing contents.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'x'</span></code>  - Write or raise FileExistsError if file exists.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'a'</span></code>  - Append. Creates new file if it doesn't exist.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'w+'</span></code> - Read and write. Deletes existing contents.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'r+'</span></code> - Read and write from the start of the file.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'a+'</span></code> - Read and write from the end of the file.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'rb'</span></code> - Read bytes from the file. Also <code class="python hljs"><span class="hljs-string">'wb'</span></code>, etc.</strong></li>
</ul><div><h3 id="exceptions-1">Exceptions</h3><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'FileNotFoundError'</span></code> can be raised when reading with <code class="python hljs"><span class="hljs-string">'r'</span></code> or <code class="python hljs"><span class="hljs-string">'r+'</span></code>.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'FileExistsError'</span></code> exception can be raised when writing with <code class="python hljs"><span class="hljs-string">'x'</span></code>.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'IsADirectoryError'</span></code> and <code class="python hljs"><span class="hljs-string">'PermissionError'</span></code> can be raised by any.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'OSError'</span></code> is the parent class of all listed exceptions.</strong></li>
</ul><div><h3 id="fileobject">File Object</h3><pre><code class="python language-python hljs">&lt;file&gt;.seek(<span class="hljs-number">0</span>)                      <span class="hljs-comment"># Moves current position to the start of file.</span>
&lt;file&gt;.seek(offset)                 <span class="hljs-comment"># Moves 'offset' chars/bytes from the start.</span>
&lt;file&gt;.seek(<span class="hljs-number">0</span>, <span class="hljs-number">2</span>)                   <span class="hljs-comment"># Moves current position to the end of file.</span>
&lt;bin_file&gt;.seek(±offset, origin)    <span class="hljs-comment"># Origin: 0 start, 1 current position, 2 end.</span>
</code></pre></div></div></div>





<pre><code class="python language-python hljs">&lt;str/bytes&gt; = &lt;file&gt;.read(size=<span class="hljs-number">-1</span>)  <span class="hljs-comment"># Reads 'size' chars/bytes or until the EOF.</span>
&lt;str/bytes&gt; = &lt;file&gt;.readline()     <span class="hljs-comment"># Returns a line or empty string/bytes on EOF.</span>
&lt;list&gt;      = &lt;file&gt;.readlines()    <span class="hljs-comment"># Returns remaining lines. Also list(&lt;file&gt;).</span>
&lt;str/bytes&gt; = next(&lt;file&gt;)          <span class="hljs-comment"># Returns a line using a read-ahead buffer.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;file&gt;.write(&lt;str/bytes&gt;)           <span class="hljs-comment"># Writes a str or bytes object to write buffer.</span>
&lt;file&gt;.writelines(&lt;collection&gt;)     <span class="hljs-comment"># Writes a coll. of strings or bytes objects.</span>
&lt;file&gt;.flush()                      <span class="hljs-comment"># Flushes write buffer. Runs every 4096/8192 B.</span>
&lt;file&gt;.close()                      <span class="hljs-comment"># Closes the file after flushing write buffer.</span>
</code></pre>
<ul>
<li><strong>Methods do not add or strip trailing newlines, not even writelines().</strong></li>
</ul>
<div><h3 id="readtextfromfile">Read Text from File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, encoding=<span class="hljs-string">'utf-8'</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> file.readlines()
</code></pre></div>

<div><h3 id="writetexttofile">Write Text to File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_to_file</span><span class="hljs-params">(filename, text)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'w'</span>, encoding=<span class="hljs-string">'utf-8'</span>) <span class="hljs-keyword">as</span> file:
        file.write(text)
</code></pre></div>

<div><h2 id="paths"><a href="#paths" name="paths">#</a>Paths</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> os, glob
<span class="hljs-keyword">from</span> pathlib <span class="hljs-keyword">import</span> Path
</code></pre></div>

<pre><code class="python language-python hljs">&lt;str&gt;  = os.getcwd()                <span class="hljs-comment"># Returns working dir. Starts as shell's $PWD.</span>
&lt;str&gt;  = os.path.join(&lt;path&gt;, ...)  <span class="hljs-comment"># Uses os.sep to join strings or Path objects.</span>
&lt;str&gt;  = os.path.realpath(&lt;path&gt;)   <span class="hljs-comment"># Resolves symlinks and calls path.abspath().</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;str&gt;  = os.path.basename(&lt;path&gt;)   <span class="hljs-comment"># Returns final component (filename or dirname).</span>
&lt;str&gt;  = os.path.dirname(&lt;path&gt;)    <span class="hljs-comment"># Returns the path without its final component.</span>
&lt;tup.&gt; = os.path.splitext(&lt;path&gt;)   <span class="hljs-comment"># Splits on last period of the final component.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt; = os.listdir(path=<span class="hljs-string">'.'</span>)       <span class="hljs-comment"># Returns all file/dirnames located at the path.</span>
&lt;list&gt; = glob.glob(<span class="hljs-string">'&lt;pattern&gt;'</span>)     <span class="hljs-comment"># Returns paths matching the wildcard pattern.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;bool&gt; = os.path.exists(&lt;path&gt;)     <span class="hljs-comment"># Checks if path exists. Also &lt;Path&gt;.exists().</span>
&lt;bool&gt; = os.path.isfile(&lt;path&gt;)     <span class="hljs-comment"># Also &lt;Path&gt;.is_file() and &lt;DirEntry&gt;.is_file().</span>
&lt;bool&gt; = os.path.isdir(&lt;path&gt;)      <span class="hljs-comment"># Also &lt;Path&gt;.is_dir() and &lt;DirEntry&gt;.is_dir().</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;stat&gt; = os.stat(&lt;path&gt;)            <span class="hljs-comment"># A status object. Also &lt;Path/DirEntry&gt;.stat().</span>
&lt;num&gt;  = &lt;stat&gt;.st_size/st_mtime/…  <span class="hljs-comment"># Returns size in bytes, modification time, etc.</span>
</code></pre>
<div><h3 id="direntry">DirEntry</h3><p><strong>Unlike listdir(), scandir() returns DirEntry objects that cache isfile, isdir, and on Windows also stat information, thus significantly increasing the performance of code that requires it.</strong></p><pre><code class="python language-python hljs">&lt;iter&gt; = os.scandir(path=<span class="hljs-string">'.'</span>)       <span class="hljs-comment"># Returns DirEntry objects located at the path.</span>
&lt;str&gt;  = &lt;DirEntry&gt;.path            <span class="hljs-comment"># Is absolute if 'path' argument was absolute.</span>
&lt;str&gt;  = &lt;DirEntry&gt;.name            <span class="hljs-comment"># Returns path's final component as a string.</span>
&lt;file&gt; = open(&lt;DirEntry&gt;)           <span class="hljs-comment"># Opens the file and returns its file object.</span>
</code></pre></div>


<div><h3 id="pathobject">Path Object</h3><pre><code class="python language-python hljs">&lt;Path&gt; = Path(&lt;path&gt; [, ...])       <span class="hljs-comment"># Accepts strings, Paths, and DirEntry objects.</span>
&lt;Path&gt; = &lt;path&gt; / &lt;path&gt; [/ ...]    <span class="hljs-comment"># First or second path must be a Path object.</span>
&lt;Path&gt; = &lt;Path&gt;.resolve()           <span class="hljs-comment"># Returns absolute path with resolved symlinks.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;Path&gt; = Path()                     <span class="hljs-comment"># Returns current working dir. Also Path('.').</span>
&lt;Path&gt; = Path.cwd()                 <span class="hljs-comment"># Returns absolute CWD. Also Path().resolve().</span>
&lt;Path&gt; = Path.home()                <span class="hljs-comment"># Returns the user's absolute home directory.</span>
&lt;Path&gt; = Path(__file__).resolve()   <span class="hljs-comment"># Returns module's path if CWD wasn't changed.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Path&gt; = &lt;Path&gt;.parent              <span class="hljs-comment"># Returns the path without its final component.</span>
&lt;str&gt;  = &lt;Path&gt;.name                <span class="hljs-comment"># Returns final component (filename or dirname).</span>
&lt;str&gt;  = &lt;Path&gt;.suffix              <span class="hljs-comment"># Returns the name's last extension, e.g. '.gz'.</span>
&lt;str&gt;  = &lt;Path&gt;.stem                <span class="hljs-comment"># Returns the name without its last extension.</span>
&lt;tup.&gt; = &lt;Path&gt;.parts               <span class="hljs-comment"># Returns a tuple of all components as strings.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;iter&gt; = &lt;Path&gt;.iterdir()           <span class="hljs-comment"># Returns directory contents as Path objects.</span>
&lt;iter&gt; = &lt;Path&gt;.glob(<span class="hljs-string">'&lt;pattern&gt;'</span>)   <span class="hljs-comment"># Returns Paths matching the wildcard pattern.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;str&gt;  = str(&lt;Path&gt;)                <span class="hljs-comment"># Returns path as string. Also &lt;Path&gt;.as_uri().</span>
&lt;file&gt; = open(&lt;Path&gt;)               <span class="hljs-comment"># Also &lt;Path&gt;.read_text/write_bytes(&lt;args&gt;).</span>
</code></pre>
<div><h2 id="oscommands"><a href="#oscommands" name="oscommands">#</a>OS Commands</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> os, shutil, subprocess
</code></pre></div>

<pre><code class="python language-python hljs">os.chdir(&lt;path&gt;)                 <span class="hljs-comment"># Changes the current working directory (CWD).</span>
os.mkdir(&lt;path&gt;, mode=<span class="hljs-number">0o777</span>)     <span class="hljs-comment"># Creates a directory. Permissions are in octal.</span>
os.makedirs(&lt;path&gt;, mode=<span class="hljs-number">0o777</span>)  <span class="hljs-comment"># Creates all path's dirs. Also `exist_ok=False`.</span>
</code></pre>
<pre><code class="python language-python hljs">shutil.copy(from, to)            <span class="hljs-comment"># Copies the file ('to' can exist or be a dir).</span>
shutil.copy2(from, to)           <span class="hljs-comment"># Also copies creation and modification time.</span>
shutil.copytree(from, to)        <span class="hljs-comment"># Copies the directory ('to' must not exist).</span>
</code></pre>
<pre><code class="python language-python hljs">os.rename(from, to)              <span class="hljs-comment"># Renames or moves the file or directory 'from'.</span>
os.replace(from, to)             <span class="hljs-comment"># Same, but overwrites file 'to' even on Windows.</span>
shutil.move(from, to)            <span class="hljs-comment"># Rename() that moves into 'to' if it's a dir.</span>
</code></pre>
<pre><code class="python language-python hljs">os.remove(&lt;path&gt;)                <span class="hljs-comment"># Deletes file. Also `$ pip3 install send2trash`.</span>
os.rmdir(&lt;path&gt;)                 <span class="hljs-comment"># Deletes the empty directory or raises OSError.</span>
shutil.rmtree(&lt;path&gt;)            <span class="hljs-comment"># Deletes the directory and all of its contents.</span>
</code></pre>
<ul>
<li><strong>Paths can be either strings, Path objects, or DirEntry objects.</strong></li>
<li><strong>Functions report OS related errors by raising OSError or one of its <a href="#exceptions-1">subclasses</a>.</strong></li>
</ul>
<div><h3 id="shellcommands">Shell Commands</h3><pre><code class="python language-python hljs">&lt;pipe&gt; = os.popen(<span class="hljs-string">'&lt;commands&gt;'</span>)  <span class="hljs-comment"># Executes commands in sh/cmd. Also os.system().</span>
&lt;str&gt;  = &lt;pipe&gt;.read(size=<span class="hljs-number">-1</span>)    <span class="hljs-comment"># Returns a combined stdout. Also readline/s().</span>
&lt;int&gt;  = &lt;pipe&gt;.close()          <span class="hljs-comment"># Returns None if last command exited with 0.</span>
</code></pre></div>

<div><h4 id="sends11tothebasiccalculatorandcapturesitsoutput">Sends "1 + 1" to the basic calculator and captures its output:</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>subprocess.run(<span class="hljs-string">'bc'</span>, input=<span class="hljs-string">'1 + 1\n'</span>, capture_output=<span class="hljs-keyword">True</span>, text=<span class="hljs-keyword">True</span>)
CompletedProcess(args=<span class="hljs-string">'bc'</span>, returncode=<span class="hljs-number">0</span>, stdout=<span class="hljs-string">'2\n'</span>, stderr=<span class="hljs-string">''</span>)
</code></pre></div>

<div><h4 id="sendstestintothebasiccalculatorrunninginstandardmodeandsavesitsoutputtotestout">Sends test.in to the basic calculator running in standard mode and saves its output to test.out:</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> shlex <span class="hljs-keyword">import</span> split
<span class="hljs-meta">&gt;&gt;&gt; </span>os.popen(<span class="hljs-string">'echo 1 + 1 &gt; test.in'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>subprocess.run(split(<span class="hljs-string">'bc -s'</span>), stdin=open(<span class="hljs-string">'test.in'</span>), stdout=open(<span class="hljs-string">'test.out'</span>, <span class="hljs-string">'w'</span>))
CompletedProcess(args=[<span class="hljs-string">'bc'</span>, <span class="hljs-string">'-s'</span>], returncode=<span class="hljs-number">0</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>open(<span class="hljs-string">'test.out'</span>).read()
<span class="hljs-string">'2\n'</span>
</code></pre></div>

<div><h2 id="json"><a href="#json" name="json">#</a>JSON</h2><p><strong>Text file format for storing collections of strings and numbers.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> json
&lt;str&gt;  = json.dumps(&lt;list/dict&gt;)  <span class="hljs-comment"># Converts collection to JSON string.</span>
&lt;coll&gt; = json.loads(&lt;str&gt;)        <span class="hljs-comment"># Converts JSON string to collection.</span>
</code></pre></div>


<div><h3 id="readcollectionfromjsonfile">Read Collection from JSON File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_json_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, encoding=<span class="hljs-string">'utf-8'</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> json.load(file)
</code></pre></div>

<div><h3 id="writecollectiontojsonfile">Write Collection to JSON File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_to_json_file</span><span class="hljs-params">(filename, collection)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'w'</span>, encoding=<span class="hljs-string">'utf-8'</span>) <span class="hljs-keyword">as</span> file:
        json.dump(collection, file, ensure_ascii=<span class="hljs-keyword">False</span>, indent=<span class="hljs-number">2</span>)
</code></pre></div>

<div><h2 id="pickle"><a href="#pickle" name="pickle">#</a>Pickle</h2><p><strong>Binary file format for storing Python objects.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> pickle
&lt;bytes&gt;  = pickle.dumps(&lt;object&gt;)  <span class="hljs-comment"># Converts object to bytes object.</span>
&lt;object&gt; = pickle.loads(&lt;bytes&gt;)   <span class="hljs-comment"># Converts bytes object to object.</span>
</code></pre></div>


<div><h3 id="readobjectfrompicklefile">Read Object from Pickle File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_pickle_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'rb'</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> pickle.load(file)
</code></pre></div>

<div><h3 id="writeobjecttopicklefile">Write Object to Pickle File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_to_pickle_file</span><span class="hljs-params">(filename, an_object)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'wb'</span>) <span class="hljs-keyword">as</span> file:
        pickle.dump(an_object, file)
</code></pre></div>

<div><h2 id="csv"><a href="#csv" name="csv">#</a>CSV</h2><p><strong>Text file format for storing spreadsheets.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> csv
</code></pre></div>


<pre><code class="python language-python hljs">&lt;file&gt;   = open(&lt;path&gt;, newline=<span class="hljs-string">''</span>)             <span class="hljs-comment"># Opens the CSV (text) file for reading.</span>
&lt;reader&gt; = csv.reader(&lt;file&gt;, dialect=<span class="hljs-string">'excel'</span>)  <span class="hljs-comment"># Also `delimiter=','`. See Parameters.</span>
&lt;list&gt;   = next(&lt;reader&gt;)                       <span class="hljs-comment"># Returns next row as a list of strings.</span>
&lt;list&gt;   = list(&lt;reader&gt;)                       <span class="hljs-comment"># Returns a list of all remaining rows.</span>
</code></pre>
<ul>
<li><strong>Without the <code class="python hljs"><span class="hljs-string">'newline=""'</span></code> argument, every '\r\n' sequence that is embedded inside a quoted field will get converted to '\n'! For details about newline argument see <a href="#open">Open</a>.</strong></li>
<li><strong>To print the spreadsheet to the console use either <a href="#table">Tabulate</a> or PrettyTable library.</strong></li>
<li><strong>For XML and binary Excel files (extensions xlsx, xlsm and xlsb) use <a href="#fileformats">Pandas</a> library.</strong></li>
<li><strong>Reader accepts any iterator (or collection) of strings, not just text files.</strong></li>
</ul>
<div><h3 id="write">Write</h3><pre><code class="python language-python hljs">&lt;file&gt;   = open(&lt;path&gt;, mode=<span class="hljs-string">'a'</span>, newline=<span class="hljs-string">''</span>)   <span class="hljs-comment"># Opens the CSV (text) file for writing.</span>
&lt;writer&gt; = csv.writer(&lt;file&gt;, dialect=<span class="hljs-string">'excel'</span>)  <span class="hljs-comment"># Also `delimiter=','`. See Parameters.</span>
&lt;writer&gt;.writerow(&lt;collection&gt;)                 <span class="hljs-comment"># Encodes each object using `str(&lt;el&gt;)`.</span>
&lt;writer&gt;.writerows(&lt;coll_of_coll&gt;)              <span class="hljs-comment"># Appends multiple rows to opened file.</span>
</code></pre></div>

<ul>
<li><strong>If file is opened without the <code class="python hljs"><span class="hljs-string">'newline=""'</span></code> argument, '\r' will be added in front of every '\n' on platforms that use '\r\n' line endings (i.e., newlines may get doubled on Windows)!</strong></li>
<li><strong>Open existing file with <code class="python hljs"><span class="hljs-string">'mode="a"'</span></code> to append to it or <code class="python hljs"><span class="hljs-string">'mode="w"'</span></code> to overwrite it.</strong></li>
</ul>
<div><h3 id="parameters">Parameters</h3><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'dialect'</span></code> - Master parameter that sets the default values. String or a 'csv.Dialect' object.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'delimiter'</span></code> - A one-character string that separates fields (comma, tab, semicolon, etc.).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'lineterminator'</span></code> - Sets how writer terminates rows. Reader looks for '\n', '\r' and '\r\n'.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'quotechar'</span></code> - Character for quoting fields containing delimiters, quotechars, '\n' or '\r'.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'escapechar'</span></code> - Character for escaping quotechars (not needed if doublequote is True).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'doublequote'</span></code> - Whether quotechars inside fields are/get doubled instead of escaped.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'quoting'</span></code> - 0: As necessary, 1: All, 2: All but numbers which are read as floats, 3: None.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'skipinitialspace'</span></code> - Is space character at the start of the field stripped by the reader.</strong></li>
</ul><div><h3 id="dialects">Dialects</h3><pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┓
┃                  │     excel    │   excel-tab  │     unix     ┃
┠──────────────────┼──────────────┼──────────────┼──────────────┨
┃ delimiter        │       ','    │      '\t'    │       ','    ┃
┃ lineterminator   │    '\r\n'    │    '\r\n'    │      '\n'    ┃
┃ quotechar        │       '"'    │       '"'    │       '"'    ┃
┃ escapechar       │      None    │      None    │      None    ┃
┃ doublequote      │      True    │      True    │      True    ┃
┃ quoting          │         0    │         0    │         1    ┃
┃ skipinitialspace │     False    │     False    │     False    ┃
┗━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┛
</code></pre></div></div>



<div><h3 id="readrowsfromcsvfile">Read Rows from CSV File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_csv_file</span><span class="hljs-params">(filename, **csv_params)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, encoding=<span class="hljs-string">'utf-8'</span>, newline=<span class="hljs-string">''</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> list(csv.reader(file, **csv_params))
</code></pre></div>

<div><h3 id="writerowstocsvfile">Write Rows to CSV File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_to_csv_file</span><span class="hljs-params">(filename, rows, mode=<span class="hljs-string">'w'</span>, **csv_params)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, mode, encoding=<span class="hljs-string">'utf-8'</span>, newline=<span class="hljs-string">''</span>) <span class="hljs-keyword">as</span> file:
        writer = csv.writer(file, **csv_params)
        writer.writerows(rows)
</code></pre></div>

<div><h2 id="sqlite"><a href="#sqlite" name="sqlite">#</a>SQLite</h2><p><strong>A server-less database engine that stores each database into its own file.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> sqlite3
&lt;conn&gt; = sqlite3.connect(&lt;path&gt;)               <span class="hljs-comment"># Opens existing or new file. Also ':memory:'.</span>
&lt;conn&gt;.close()                                 <span class="hljs-comment"># Closes connection. Discards uncommitted data.</span>
</code></pre></div>


<div><h3 id="read">Read</h3><pre><code class="python language-python hljs">&lt;cursor&gt; = &lt;conn&gt;.execute(<span class="hljs-string">'&lt;query&gt;'</span>)           <span class="hljs-comment"># Can raise a subclass of the sqlite3.Error.</span>
&lt;tuple&gt;  = &lt;cursor&gt;.fetchone()                 <span class="hljs-comment"># Returns the next row. Also next(&lt;cursor&gt;).</span>
&lt;list&gt;   = &lt;cursor&gt;.fetchall()                 <span class="hljs-comment"># Returns remaining rows. Also list(&lt;cursor&gt;).</span>
</code></pre></div>

<div><h3 id="write-1">Write</h3><pre><code class="python language-python hljs">&lt;conn&gt;.execute(<span class="hljs-string">'&lt;query&gt;'</span>)                      <span class="hljs-comment"># Can raise a subclass of the sqlite3.Error.</span>
&lt;conn&gt;.commit()                                <span class="hljs-comment"># Saves all changes since the last commit.</span>
&lt;conn&gt;.rollback()                              <span class="hljs-comment"># Discards all changes since the last commit.</span>
</code></pre></div>

<div><h4 id="or">Or:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">with</span> &lt;conn&gt;:                                   <span class="hljs-comment"># Exits the block with commit() or rollback(),</span>
    &lt;conn&gt;.execute(<span class="hljs-string">'&lt;query&gt;'</span>)                  <span class="hljs-comment"># depending on whether any exception occurred.</span>
</code></pre></div>

<div><h3 id="placeholders">Placeholders</h3><pre><code class="python language-python hljs">&lt;conn&gt;.execute(<span class="hljs-string">'&lt;query&gt;'</span>, &lt;list/tuple&gt;)        <span class="hljs-comment"># Replaces every question mark with an item.</span>
&lt;conn&gt;.execute(<span class="hljs-string">'&lt;query&gt;'</span>, &lt;dict/namedtuple&gt;)   <span class="hljs-comment"># Replaces every :&lt;key&gt; with a matching value.</span>
&lt;conn&gt;.executemany(<span class="hljs-string">'&lt;query&gt;'</span>, &lt;coll_of_coll&gt;)  <span class="hljs-comment"># Executes the query once for each collection.</span>
</code></pre></div>

<ul>
<li><strong>Passed values can be of type str, int, float, bytes, None, or bool (stored as 1 or 0).</strong></li>
<li><strong>SQLite does not restrict columns to any type unless table is declared as strict.</strong></li>
</ul>
<div><h3 id="example-1">Example</h3><p><strong>Values are not actually saved in this example because <code class="python hljs"><span class="hljs-string">'conn.commit()'</span></code> is omitted!</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>conn = sqlite3.connect(<span class="hljs-string">'test.db'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>conn.execute(<span class="hljs-string">'CREATE TABLE person (name TEXT, height INTEGER) STRICT'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>conn.execute(<span class="hljs-string">'INSERT INTO person VALUES (?, ?)'</span>, (<span class="hljs-string">'Jean-Luc'</span>, <span class="hljs-number">187</span>))
<span class="hljs-meta">&gt;&gt;&gt; </span>conn.execute(<span class="hljs-string">'SELECT rowid, * FROM person'</span>).fetchall()
[(<span class="hljs-number">1</span>, <span class="hljs-string">'Jean-Luc'</span>, <span class="hljs-number">187</span>)]
</code></pre></div>


<div><h3 id="sqlalchemy">SQLAlchemy</h3><p><strong>Library for interacting with various DB systems via SQL, method chaining, or ORM.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install sqlalchemy</span>
<span class="hljs-keyword">from</span> sqlalchemy <span class="hljs-keyword">import</span> create_engine, text
&lt;engine&gt; = create_engine(<span class="hljs-string">'&lt;url&gt;'</span>)              <span class="hljs-comment"># Url: 'dialect://user:password@host/dbname'.</span>
&lt;conn&gt;   = &lt;engine&gt;.connect()                  <span class="hljs-comment"># Creates a connection. Also &lt;conn&gt;.close().</span>
&lt;cursor&gt; = &lt;conn&gt;.execute(text(<span class="hljs-string">'&lt;query&gt;'</span>), …)  <span class="hljs-comment"># `&lt;dict&gt;`. Replaces every :&lt;key&gt; with value.</span>
<span class="hljs-keyword">with</span> &lt;conn&gt;.begin(): ...                       <span class="hljs-comment"># Exits the block with a commit or rollback.</span>
</code></pre></div>


<pre><code class="text language-text">┏━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Dialect         │ pip3 install │           Dependencies           ┃
┠─────────────────┼──────────────┼──────────────────────────────────┨
┃ mysql           │ mysqlclient  │ www.pypi.org/project/mysqlclient ┃
┃ postgresql      │ psycopg2     │ www.pypi.org/project/psycopg2    ┃
┃ mssql           │ pyodbc       │ www.pypi.org/project/pyodbc      ┃
┃ oracle+oracledb │ oracledb     │ www.pypi.org/project/oracledb    ┃
┗━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
</code></pre>
<div><h2 id="bytes"><a href="#bytes" name="bytes">#</a>Bytes</h2><p><strong>A bytes object is an immutable sequence of single bytes. Mutable version is called bytearray.</strong></p><pre><code class="python language-python hljs">&lt;bytes&gt; = <span class="hljs-string">b'&lt;str&gt;'</span>                       <span class="hljs-comment"># Only accepts ASCII chars and [\x00-\xff].</span>
&lt;int&gt;   = &lt;bytes&gt;[index]                 <span class="hljs-comment"># Returns an integer in range from 0 to 255.</span>
&lt;bytes&gt; = &lt;bytes&gt;[&lt;slice&gt;]               <span class="hljs-comment"># Returns bytes even if it has one element.</span>
&lt;bytes&gt; = &lt;bytes&gt;.join(&lt;coll_of_bytes&gt;)  <span class="hljs-comment"># Joins elements using bytes as a separator.</span>
</code></pre></div>


<div><h3 id="encode-1">Encode</h3><pre><code class="python language-python hljs">&lt;bytes&gt; = bytes(&lt;coll_of_ints&gt;)          <span class="hljs-comment"># Integers must be in range from 0 to 255.</span>
&lt;bytes&gt; = bytes(&lt;str&gt;, <span class="hljs-string">'utf-8'</span>)          <span class="hljs-comment"># Encodes the string. Also &lt;str&gt;.encode().</span>
&lt;bytes&gt; = bytes.fromhex(<span class="hljs-string">'&lt;hex&gt;'</span>)         <span class="hljs-comment"># Hex pairs can be separated by whitespace.</span>
&lt;bytes&gt; = &lt;int&gt;.to_bytes(n_bytes, …)     <span class="hljs-comment"># `byteorder='big/little', signed=False`.</span>
</code></pre></div>

<div><h3 id="decode-1">Decode</h3><pre><code class="python language-python hljs">&lt;list&gt;  = list(&lt;bytes&gt;)                  <span class="hljs-comment"># Returns integers in range from 0 to 255.</span>
&lt;str&gt;   = str(&lt;bytes&gt;, <span class="hljs-string">'utf-8'</span>)          <span class="hljs-comment"># Returns a string. Also &lt;bytes&gt;.decode().</span>
&lt;str&gt;   = &lt;bytes&gt;.hex()                  <span class="hljs-comment"># Returns hex pairs. Accepts `sep=&lt;str&gt;`.</span>
&lt;int&gt;   = int.from_bytes(&lt;bytes&gt;, …)     <span class="hljs-comment"># `byteorder='big/little', signed=False`.</span>
</code></pre></div>

<div><h3 id="readbytesfromfile">Read Bytes from File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_bytes</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'rb'</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> file.read()
</code></pre></div>

<div><h3 id="writebytestofile">Write Bytes to File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_bytes</span><span class="hljs-params">(filename, bytes_obj)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'wb'</span>) <span class="hljs-keyword">as</span> file:
        file.write(bytes_obj)
</code></pre></div>

<div><h2 id="struct"><a href="#struct" name="struct">#</a>Struct</h2><ul>
<li><strong>Module that performs conversions between a sequence of numbers and a bytes object.</strong></li>
<li><strong>System’s type sizes, byte order, and alignment rules are used by default.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> struct <span class="hljs-keyword">import</span> pack, unpack

&lt;bytes&gt; = pack(<span class="hljs-string">'&lt;format&gt;'</span>, &lt;el_1&gt; [, ...])  <span class="hljs-comment"># Packs numbers according to format string.</span>
&lt;tuple&gt; = unpack(<span class="hljs-string">'&lt;format&gt;'</span>, &lt;bytes&gt;)       <span class="hljs-comment"># Use iter_unpack() to get iter of tuples.</span>
</code></pre></div>


<pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>pack(<span class="hljs-string">'&gt;hhl'</span>, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
<span class="hljs-string">b'\x00\x01\x00\x02\x00\x00\x00\x03'</span>
<span class="hljs-meta">&gt;&gt;&gt; </span>unpack(<span class="hljs-string">'&gt;hhl'</span>, <span class="hljs-string">b'\x00\x01\x00\x02\x00\x00\x00\x03'</span>)
(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>)
</code></pre>
<h3 id="format-2">Format</h3><div><h4 id="forstandardtypesizesandmanualalignmentpaddingstartformatstringwith">For standard type sizes and manual alignment (padding) start format string with:</h4><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'='</span></code> - System's byte order (usually little-endian).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'&lt;'</span></code> - Little-endian (i.e. least significant byte first).</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'&gt;'</span></code> - Big-endian (also <code class="python hljs"><span class="hljs-string">'!'</span></code>).</strong></li>
</ul><div><h4 id="besidesnumberspackandunpackalsosupportbytesobjectsasapartofthesequence">Besides numbers, pack() and unpack() also support bytes objects as a part of the sequence:</h4><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'c'</span></code> - A bytes object with a single element. For pad byte use <code class="python hljs"><span class="hljs-string">'x'</span></code>.</strong></li>
<li><strong><code class="apache hljs"><span class="hljs-section">'&lt;n&gt;s'</span><span class="hljs-attribute"></span></code> - A bytes object with n elements (not effected by byte order).</strong></li>
</ul></div></div><div><div><h4 id="integertypesuseacapitalletterforunsignedtypeminimumandstandardsizesareinbrackets">Integer types. Use a capital letter for unsigned type. Minimum and standard sizes are in brackets:</h4><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'b'</span></code> - char (1/1)</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'h'</span></code> - short (2/2)</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'i'</span></code> - int (2/4)</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'l'</span></code> - long (4/4)</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'q'</span></code> - long long (8/8)</strong></li>
</ul></div><div><h4 id="floatingpointtypesstructalwaysusesstandardsizes">Floating point types (struct always uses standard sizes):</h4><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'f'</span></code> - float (4/4)</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'d'</span></code> - double (8/8)</strong></li>
</ul></div></div>








<div><h2 id="array"><a href="#array" name="array">#</a>Array</h2><p><strong>List that can only hold numbers of a predefined type. Available types and their minimum sizes in bytes are listed above. Type sizes and byte order are always determined by the sys­tem,&nbsp;however bytes of each element can be reversed (by calling the byteswap() method).</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> array <span class="hljs-keyword">import</span> array
</code></pre></div>


<pre><code class="python language-python hljs">&lt;array&gt; = array(<span class="hljs-string">'&lt;typecode&gt;'</span> [, &lt;coll&gt;])  <span class="hljs-comment"># Creates array. Accepts collection of numbers.</span>
&lt;array&gt; = array(<span class="hljs-string">'&lt;typecode&gt;'</span>, &lt;bytes&gt;)    <span class="hljs-comment"># Copies passed bytes into the array's memory.</span>
&lt;array&gt; = array(<span class="hljs-string">'&lt;typecode&gt;'</span>, &lt;array&gt;)    <span class="hljs-comment"># Treats passed array as a sequence of numbers.</span>
&lt;array&gt;.fromfile(&lt;file&gt;, n_items)         <span class="hljs-comment"># Appends file contents to the array's memory.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;bytes&gt; = bytes(&lt;array&gt;)                  <span class="hljs-comment"># Returns a copy of array's memory as bytes.</span>
&lt;file&gt;.write(&lt;array&gt;)                     <span class="hljs-comment"># Appends array's memory to the binary file.</span>
</code></pre>
<div><h2 id="memoryview"><a href="#memoryview" name="memoryview">#</a>Memory View</h2><p><strong>A sequence object that points to the memory of another bytes-like object. Each element can reference a single or multiple consecutive bytes, depending on format. Order and number of elements can be changed with slicing.</strong></p><pre><code class="python language-python hljs">&lt;mview&gt; = memoryview(&lt;bytes/array&gt;)       <span class="hljs-comment"># Returns mutable memoryview if array is passed.</span>
&lt;obj&gt;   = &lt;mview&gt;[index]                  <span class="hljs-comment"># Returns an int/float. Bytes if format is 'c'.</span>
&lt;mview&gt; = &lt;mview&gt;[&lt;slice&gt;]                <span class="hljs-comment"># Returns memoryview with rearranged elements.</span>
&lt;mview&gt; = &lt;mview&gt;.cast(<span class="hljs-string">'&lt;typecode&gt;'</span>)      <span class="hljs-comment"># Only works between B/b/c and the other types.</span>
&lt;mview&gt;.release()                         <span class="hljs-comment"># Releases the memory buffer of the base object.</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;bytes&gt; = bytes(&lt;mview&gt;)                  <span class="hljs-comment"># Returns a new bytes object. Also bytearray().</span>
&lt;bytes&gt; = &lt;bytes&gt;.join(&lt;coll_of_mviews&gt;)  <span class="hljs-comment"># Joins memoryviews using bytes as a separator.</span>
&lt;array&gt; = array(<span class="hljs-string">'&lt;typecode&gt;'</span>, &lt;mview&gt;)    <span class="hljs-comment"># Treats passed mview as a sequence of numbers.</span>
&lt;file&gt;.write(&lt;mview&gt;)                     <span class="hljs-comment"># Appends `bytes(&lt;mview&gt;)` to the binary file.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;list&gt;  = list(&lt;mview&gt;)                   <span class="hljs-comment"># Returns list of ints, floats or bytes objects.</span>
&lt;str&gt;   = str(&lt;mview&gt;, <span class="hljs-string">'utf-8'</span>)           <span class="hljs-comment"># Treats passed memoryview as `bytes(&lt;mview&gt;)`.</span>
&lt;str&gt;   = &lt;mview&gt;.hex()                   <span class="hljs-comment"># Returns hexadecimal pairs. Also `sep=&lt;str&gt;`.</span>
</code></pre>
<div><h2 id="deque"><a href="#deque" name="deque">#</a>Deque</h2><p><strong>List with efficient appends and pops from either side.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> deque
</code></pre></div>


<pre><code class="python language-python hljs">&lt;deque&gt; = deque(&lt;collection&gt;)     <span class="hljs-comment"># Use `maxlen=&lt;int&gt;` to set size limit.</span>
&lt;deque&gt;.appendleft(&lt;el&gt;)          <span class="hljs-comment"># Opposite element is dropped if full.</span>
&lt;deque&gt;.extendleft(&lt;collection&gt;)  <span class="hljs-comment"># Prepends reversed coll. to the deque.</span>
&lt;deque&gt;.rotate(n=<span class="hljs-number">1</span>)               <span class="hljs-comment"># Last element becomes the first one.</span>
&lt;el&gt; = &lt;deque&gt;.popleft()          <span class="hljs-comment"># Raises IndexError if deque is empty.</span>
</code></pre>
<div><h2 id="operator"><a href="#operator" name="operator">#</a>Operator</h2><p><strong>Module of functions that provide the functionality of operators. Functions are grouped by operator precedence, from least to most binding. Functions and operators in first, third and fifth line are also ordered by precedence within a group.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> operator <span class="hljs-keyword">as</span> op
</code></pre></div>


<pre><code class="python language-python hljs">&lt;bool&gt; = op.not_(&lt;obj&gt;)                                        <span class="hljs-comment"># or, and, not (or/and missing)</span>
&lt;bool&gt; = op.eq/ne/lt/ge/is_/is_not/contains(&lt;obj&gt;, &lt;obj&gt;)      <span class="hljs-comment"># ==, !=, &lt;, &gt;=, is, is not, in</span>
&lt;obj&gt;  = op.or_/xor/and_(&lt;int/set&gt;, &lt;int/set&gt;)                 <span class="hljs-comment"># |, ^, &amp; (sorted by precedence)</span>
&lt;int&gt;  = op.lshift/rshift(&lt;int&gt;, &lt;int&gt;)                        <span class="hljs-comment"># &lt;&lt;, &gt;&gt; (i.e. &lt;int&gt; &lt;&lt; n_bits)</span>
&lt;obj&gt;  = op.add/sub/mul/truediv/floordiv/mod(&lt;obj&gt;, &lt;obj&gt;)     <span class="hljs-comment"># +, -, *, /, //, % (two groups)</span>
&lt;num&gt;  = op.neg/invert(&lt;num&gt;)                                  <span class="hljs-comment"># -, ~ (negate and bitwise not)</span>
&lt;num&gt;  = op.pow(&lt;num&gt;, &lt;num&gt;)                                  <span class="hljs-comment"># ** (pow() accepts 3 arguments)</span>
&lt;func&gt; = op.itemgetter/attrgetter/methodcaller(&lt;obj&gt; [, ...])  <span class="hljs-comment"># [index/key], .name, .name([…])</span>
</code></pre>
<pre><code class="python language-python hljs">elementwise_sum  = map(op.add, list_a, list_b)
sorted_by_second = sorted(&lt;coll&gt;, key=op.itemgetter(<span class="hljs-number">1</span>))
sorted_by_both   = sorted(&lt;coll&gt;, key=op.itemgetter(<span class="hljs-number">1</span>, <span class="hljs-number">0</span>))
first_element    = op.methodcaller(<span class="hljs-string">'pop'</span>, <span class="hljs-number">0</span>)(&lt;list&gt;)
</code></pre>
<ul>
<li><strong>Most operators call the object's special method that is named after them (second object is passed as an argument), while logical operators call their own code that relies on bool().</strong></li>
<li><strong>Comparisons can be chained: <code class="python hljs"><span class="hljs-string">'x &lt; y &lt; z'</span></code> gets converted to <code class="python hljs"><span class="hljs-string">'(x &lt; y) and (y &lt; z)'</span></code>.</strong></li>
</ul>
<div><h2 id="matchstatement"><a href="#matchstatement" name="matchstatement">#</a>Match Statement</h2><p><strong>Executes the first block with matching pattern.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">match</span> &lt;object/expression&gt;:
    <span class="hljs-keyword">case</span> &lt;pattern&gt; [<span class="hljs-keyword">if</span> &lt;condition&gt;]:
        &lt;code&gt;
    ...
</code></pre></div>


<div><h3 id="patterns">Patterns</h3><pre><code class="python language-python hljs">&lt;value_pattern&gt; = <span class="hljs-number">1</span>/<span class="hljs-string">'abc'</span>/<span class="hljs-keyword">True</span>/<span class="hljs-keyword">None</span>/math.pi        <span class="hljs-comment"># Matches the literal or attribute's value.</span>
&lt;class_pattern&gt; = &lt;type&gt;()                         <span class="hljs-comment"># Matches any object of that type (or ABC).</span>
&lt;wildcard_patt&gt; = _                                <span class="hljs-comment"># Matches any object. Useful in last case.</span>
&lt;capture_patt&gt;  = &lt;name&gt;                           <span class="hljs-comment"># Matches any object and binds it to name.</span>
&lt;as_pattern&gt;    = &lt;pattern&gt; <span class="hljs-keyword">as</span> &lt;name&gt;              <span class="hljs-comment"># Binds match to name. Also &lt;type&gt;(&lt;name&gt;).</span>
&lt;or_pattern&gt;    = &lt;pattern&gt; | &lt;pattern&gt; [| ...]    <span class="hljs-comment"># Matches if any of listed patterns match.</span>
&lt;sequence_patt&gt; = [&lt;pattern&gt;, ...]                 <span class="hljs-comment"># Matches a sequence. All items must match.</span>
&lt;mapping_patt&gt;  = {&lt;value_pattern&gt;: &lt;patt&gt;, ...}   <span class="hljs-comment"># Matches a dict if it has matching items.</span>
&lt;class_pattern&gt; = &lt;type&gt;(&lt;attr_name&gt;=&lt;patt&gt;, ...)  <span class="hljs-comment"># Matches object that has matching attrbs.</span>
</code></pre></div>

<ul>
<li><strong>Sequence pattern can also be written as a tuple, either with or without the brackets.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'*&lt;name&gt;'</span></code> and <code class="python hljs"><span class="hljs-string">'**&lt;name&gt;'</span></code> in sequence/mapping patterns to bind remaining items.</strong></li>
<li><strong>Sequence pattern must match all items of the collection, while mapping pattern does not.</strong></li>
<li><strong>Patterns can be surrounded with brackets to override their precedence: <code class="python hljs"><span class="hljs-string">'|'</span></code> &gt; <code class="python hljs"><span class="hljs-string">'as'</span></code> &gt; <code class="python hljs"><span class="hljs-string">','</span></code>. For example, <code class="python hljs"><span class="hljs-string">'[1, 2]'</span></code> is matched by the <code class="python hljs"><span class="hljs-string">'case 1|2, 2|3 as x if x == 2:'</span></code> block.</strong></li>
<li><strong>All names that are bound in the matching case, as well as variables initialized in its body, are visible after the match statement (only function block delimits scope).</strong></li>
</ul>
<div><h3 id="example-2">Example</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> pathlib <span class="hljs-keyword">import</span> Path
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">match</span> Path(<span class="hljs-string">'/home/gto/python-cheatsheet/README.md'</span>):
<span class="hljs-meta">... </span>    <span class="hljs-keyword">case</span> Path(
<span class="hljs-meta">... </span>        parts=[<span class="hljs-string">'/'</span>, <span class="hljs-string">'home'</span>, user, *_]
<span class="hljs-meta">... </span>    ) <span class="hljs-keyword">as</span> p <span class="hljs-keyword">if</span> p.name.lower().startswith(<span class="hljs-string">'readme'</span>) <span class="hljs-keyword">and</span> p.is_file():
<span class="hljs-meta">... </span>        print(<span class="hljs-string">f'<span class="hljs-subst">{p.name}</span> is a readme file that belongs to user <span class="hljs-subst">{user}</span>.'</span>)
README.md is a readme file that belongs to user gto.
</code></pre></div>

<div><h2 id="logging"><a href="#logging" name="logging">#</a>Logging</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> logging <span class="hljs-keyword">as</span> log
</code></pre></div>

<pre><code class="python language-python hljs">log.basicConfig(filename=&lt;path&gt;, level=<span class="hljs-string">'DEBUG'</span>)   <span class="hljs-comment"># Configures the root logger (see Setup).</span>
log.debug/info/warning/error/critical(&lt;str&gt;)      <span class="hljs-comment"># Sends passed message to the root logger.</span>
&lt;Logger&gt; = log.getLogger(__name__)                <span class="hljs-comment"># Returns a logger named after the module.</span>
&lt;Logger&gt;.&lt;level&gt;(&lt;str&gt;)                           <span class="hljs-comment"># Sends the message. Same levels as above.</span>
&lt;Logger&gt;.exception(&lt;str&gt;)                         <span class="hljs-comment"># Error() that appends caught exception.</span>
</code></pre>
<div><h3 id="setup">Setup</h3><pre><code class="python language-python hljs">log.basicConfig(
    filename=<span class="hljs-keyword">None</span>,                                <span class="hljs-comment"># Prints to stderr or appends to file.</span>
    format=<span class="hljs-string">'%(levelname)s:%(name)s:%(message)s'</span>,  <span class="hljs-comment"># Add '%(asctime)s' for local datetime.</span>
    level=log.WARNING,                            <span class="hljs-comment"># Drops messages with a lower priority.</span>
    handlers=[log.StreamHandler(sys.stderr)]      <span class="hljs-comment"># Uses FileHandler if filename is set.</span>
)
</code></pre></div>

<pre><code class="python language-python hljs">&lt;Formatter&gt; = log.Formatter(<span class="hljs-string">'&lt;format&gt;'</span>)           <span class="hljs-comment"># Formats messages according to format.</span>
&lt;Handler&gt; = log.FileHandler(&lt;path&gt;, mode=<span class="hljs-string">'a'</span>)     <span class="hljs-comment"># Appends to file. Also `encoding=None`.</span>
&lt;Handler&gt;.setFormatter(&lt;Formatter&gt;)               <span class="hljs-comment"># Only outputs bare messages by default.</span>
&lt;Handler&gt;.setLevel(&lt;int/str&gt;)                     <span class="hljs-comment"># Prints/saves every message by default.</span>
&lt;Logger&gt;.addHandler(&lt;Handler&gt;)                    <span class="hljs-comment"># Logger can have more than one handler.</span>
&lt;Logger&gt;.setLevel(&lt;int/str&gt;)                      <span class="hljs-comment"># What's sent to its/ancestors' handlers.</span>
&lt;Logger&gt;.propagate = &lt;bool&gt;                       <span class="hljs-comment"># Cuts off ancestors' handlers if False.</span>
</code></pre>
<ul>
<li><strong>Parent logger can be specified by naming the child logger <code class="python hljs"><span class="hljs-string">'&lt;parent&gt;.&lt;name&gt;'</span></code>.</strong></li>
<li><strong>If logger doesn't have a set level, it inherits it from the first ancestor that does.</strong></li>
<li><strong>Formatter also accepts: pathname, filename, funcName, lineno, thread and process.</strong></li>
<li><strong>RotatingFileHandler creates and deletes files based on 'maxBytes', 'backupCount' args.</strong></li>
<li><strong>An object with <code class="python hljs"><span class="hljs-string">'filter(&lt;LogRecord&gt;)'</span></code> method (or the method itself) can be added to loggers and handlers via addFilter(). Message is dropped if filter() returns a false value.</strong></li>
</ul>
<div><h4 id="createsaloggerthatwritesallmessagestoafileandsendsthemtotherootshandlerthatprintswarningsorhigher">Creates a logger that writes all messages to a file and sends them to the root's handler that prints warnings or higher:</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>logger = log.getLogger(<span class="hljs-string">'my_module'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>handler = log.FileHandler(<span class="hljs-string">'test.log'</span>, encoding=<span class="hljs-string">'utf-8'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>handler.setFormatter(log.Formatter(<span class="hljs-string">'%(asctime)s %(levelname)s:%(name)s:%(message)s'</span>))
<span class="hljs-meta">&gt;&gt;&gt; </span>logger.addHandler(handler)
<span class="hljs-meta">&gt;&gt;&gt; </span>logger.setLevel(<span class="hljs-string">'DEBUG'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>log.basicConfig()
<span class="hljs-meta">&gt;&gt;&gt; </span>log.root.handlers[<span class="hljs-number">0</span>].setLevel(<span class="hljs-string">'WARNING'</span>)
<span class="hljs-meta">&gt;&gt;&gt; </span>logger.critical(<span class="hljs-string">'Running out of disk space.'</span>)
CRITICAL:my_module:Running out of disk space.
<span class="hljs-meta">&gt;&gt;&gt; </span>print(open(<span class="hljs-string">'test.log'</span>).read())
2023-02-07 23:21:01,430 CRITICAL:my_module:Running out of disk space.
</code></pre></div>

<div><h2 id="introspection"><a href="#introspection" name="introspection">#</a>Introspection</h2><pre><code class="python language-python hljs">&lt;list&gt; = dir()                      <span class="hljs-comment"># Local names of variables, functions, classes and modules.</span>
&lt;dict&gt; = vars()                     <span class="hljs-comment"># Dict of local names and their objects. Same as locals().</span>
&lt;dict&gt; = globals()                  <span class="hljs-comment"># Dict of global names and their objects, e.g. __builtin__.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;list&gt; = dir(&lt;obj&gt;)                 <span class="hljs-comment"># Returns names of object's attributes (including methods).</span>
&lt;dict&gt; = vars(&lt;obj&gt;)                <span class="hljs-comment"># Returns dict of writable attributes. Also &lt;obj&gt;.__dict__.</span>
&lt;bool&gt; = hasattr(&lt;obj&gt;, <span class="hljs-string">'&lt;name&gt;'</span>)   <span class="hljs-comment"># Checks if object possesses attribute of the passed name.</span>
value  = getattr(&lt;obj&gt;, <span class="hljs-string">'&lt;name&gt;'</span>)   <span class="hljs-comment"># Returns the object's attribute or raises AttributeError.</span>
setattr(&lt;obj&gt;, <span class="hljs-string">'&lt;name&gt;'</span>, value)     <span class="hljs-comment"># Sets attribute. Only works on objects with __dict__ attr.</span>
delattr(&lt;obj&gt;, <span class="hljs-string">'&lt;name&gt;'</span>)            <span class="hljs-comment"># Deletes attribute from __dict__. Also `del &lt;obj&gt;.&lt;name&gt;`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Sig&gt;  = inspect.signature(&lt;func&gt;)  <span class="hljs-comment"># Returns Signature object of the passed function or class.</span>
&lt;dict&gt; = &lt;Sig&gt;.parameters           <span class="hljs-comment"># Returns dict of Parameters. Also &lt;Sig&gt;.return_annotation.</span>
&lt;memb&gt; = &lt;Param&gt;.kind               <span class="hljs-comment"># Returns ParameterKind member (Parameter.KEYWORD_ONLY, …).</span>
&lt;type&gt; = &lt;Param&gt;.annotation         <span class="hljs-comment"># Returns Parameter.empty if missing. Also &lt;Param&gt;.default.</span>
</code></pre>
<div><h2 id="threading"><a href="#threading" name="threading">#</a>Threading</h2><p><strong>CPython interpreter can only run a single thread at a time. Using multiple threads won't result in a faster execution, unless at least one of the threads contains an I/O operation.</strong></p><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> threading <span class="hljs-keyword">import</span> Thread, Lock, RLock, Semaphore, Event, Barrier
<span class="hljs-keyword">from</span> concurrent.futures <span class="hljs-keyword">import</span> ThreadPoolExecutor, as_completed
</code></pre></div>


<div><h3 id="thread">Thread</h3><pre><code class="python language-python hljs">&lt;Thread&gt; = Thread(target=&lt;function&gt;)           <span class="hljs-comment"># Use `args=&lt;collection&gt;` to set the arguments.</span>
&lt;Thread&gt;.start()                               <span class="hljs-comment"># Starts the thread. Also &lt;Thread&gt;.is_alive().</span>
&lt;Thread&gt;.join()                                <span class="hljs-comment"># Waits until the thread has finished executing.</span>
</code></pre></div>

<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'kwargs=&lt;dict&gt;'</span></code> to pass keyword arguments to the function.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'daemon=True'</span></code>, or the program won't be able to exit while the thread is alive.</strong></li>
</ul>
<div><h3 id="lock">Lock</h3><pre><code class="python language-python hljs">&lt;lock&gt; = Lock/RLock()                          <span class="hljs-comment"># RLock can only be released by acquirer thread.</span>
&lt;lock&gt;.acquire()                               <span class="hljs-comment"># Blocks (waits) until lock becomes available.</span>
&lt;lock&gt;.release()                               <span class="hljs-comment"># Releases the lock so it can be acquired again.</span>
</code></pre></div>

<div><h4 id="or-1">Or:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">with</span> &lt;lock&gt;:                                   <span class="hljs-comment"># Enters the block by calling method acquire().</span>
    ...                                        <span class="hljs-comment"># Exits it by calling release(), even on error.</span>
</code></pre></div>

<div><h3 id="semaphoreeventbarrier">Semaphore, Event, Barrier</h3><pre><code class="python language-python hljs">&lt;Semaphore&gt; = Semaphore(value=<span class="hljs-number">1</span>)               <span class="hljs-comment"># Lock that can be acquired by 'value' threads.</span>
&lt;Event&gt;     = Event()                          <span class="hljs-comment"># Method wait() blocks until set() is called.</span>
&lt;Barrier&gt;   = Barrier(&lt;int&gt;)                   <span class="hljs-comment"># Wait() blocks until it is called int times.</span>
</code></pre></div>

<div><h3 id="queue">Queue</h3><pre><code class="python language-python hljs">&lt;Queue&gt; = queue.Queue(maxsize=<span class="hljs-number">0</span>)               <span class="hljs-comment"># A first-in-first-out queue. It's thread safe.</span>
&lt;Queue&gt;.put(&lt;obj&gt;)                             <span class="hljs-comment"># The call blocks until queue stops being full.</span>
&lt;Queue&gt;.put_nowait(&lt;obj&gt;)                      <span class="hljs-comment"># Raises queue.Full exception if queue is full.</span>
&lt;obj&gt; = &lt;Queue&gt;.get()                          <span class="hljs-comment"># The call blocks until queue stops being empty.</span>
&lt;obj&gt; = &lt;Queue&gt;.get_nowait()                   <span class="hljs-comment"># Raises queue.Empty exception if it is empty.</span>
</code></pre></div>

<div><h3 id="threadpoolexecutor">Thread Pool Executor</h3><pre><code class="python language-python hljs">&lt;Exec&gt; = ThreadPoolExecutor(max_workers=<span class="hljs-keyword">None</span>)  <span class="hljs-comment"># Also `with ThreadPoolExecutor() as &lt;name&gt;: …`.</span>
&lt;iter&gt; = &lt;Exec&gt;.map(&lt;func&gt;, &lt;args_1&gt;, ...)     <span class="hljs-comment"># Multithreaded and non-lazy map(). Keeps order.</span>
&lt;Futr&gt; = &lt;Exec&gt;.submit(&lt;func&gt;, &lt;arg_1&gt;, ...)   <span class="hljs-comment"># Creates a thread and returns its Future obj.</span>
&lt;Exec&gt;.shutdown()                              <span class="hljs-comment"># Waits for all the threads to finish executing.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;bool&gt; = &lt;Future&gt;.done()                       <span class="hljs-comment"># Checks if the thread has finished executing.</span>
&lt;obj&gt;  = &lt;Future&gt;.result(timeout=<span class="hljs-keyword">None</span>)         <span class="hljs-comment"># Raises TimeoutError after 'timeout' seconds.</span>
&lt;bool&gt; = &lt;Future&gt;.cancel()                     <span class="hljs-comment"># Cancels or returns False if running/finished.</span>
&lt;iter&gt; = as_completed(&lt;coll_of_Futures&gt;)       <span class="hljs-comment"># `next(&lt;iter&gt;)` returns next completed Future.</span>
</code></pre>
<ul>
<li><strong>Map() and as_completed() also accept 'timeout'. It causes futures.TimeoutError when next() is called/blocking. Map() times from original call and as_completed() from first call to next(). As_completed() fails if next() is called too late, even if all threads are done.</strong></li>
<li><strong>Exceptions that happen inside threads are raised when map iterator's next() or Future's result() are called. Future's exception() method returns an exception object or None.</strong></li>
<li><strong>ProcessPoolExecutor provides true parallelism but: everything sent to/from workers must be <a href="#pickle">pickable</a>, queues must be sent using executor's 'initargs' and 'initializer' parameters, and executor should only be reachable via <code class="python hljs"><span class="hljs-string">'if __name__ == "__main__": ...'</span></code>.</strong></li>
</ul>
<div><h2 id="coroutines"><a href="#coroutines" name="coroutines">#</a>Coroutines</h2><ul>
<li><strong>Coroutines have a lot in common with threads, but unlike threads, they only give up control when they call another coroutine and they don’t consume as much memory.</strong></li>
<li><strong>Coroutine definition starts with <code class="python hljs"><span class="hljs-string">'async'</span></code> keyword and its call with <code class="python hljs"><span class="hljs-string">'await'</span></code> keyword.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'asyncio.run(&lt;coroutine&gt;)'</span></code> to start the first/main coroutine.</strong></li>
</ul><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> asyncio <span class="hljs-keyword">as</span> aio
</code></pre></div>


<pre><code class="python language-python hljs">&lt;coro&gt; = &lt;async_function&gt;(&lt;args&gt;)          <span class="hljs-comment"># Creates a coroutine by calling async def function.</span>
&lt;obj&gt;  = <span class="hljs-keyword">await</span> &lt;coroutine&gt;                 <span class="hljs-comment"># Starts the coroutine. Returns its result or None.</span>
&lt;task&gt; = aio.create_task(&lt;coroutine&gt;)      <span class="hljs-comment"># Schedules it for execution. Always keep the task.</span>
&lt;obj&gt;  = <span class="hljs-keyword">await</span> &lt;task&gt;                      <span class="hljs-comment"># Returns coroutine's result. Also &lt;task&gt;.cancel().</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;coro&gt; = aio.gather(&lt;coro/task&gt;, ...)      <span class="hljs-comment"># Schedules coros. Returns list of results on await.</span>
&lt;coro&gt; = aio.wait(&lt;tasks&gt;, return_when=…)  <span class="hljs-comment"># `'ALL/FIRST_COMPLETED'`. Returns (done, pending).</span>
&lt;iter&gt; = aio.as_completed(&lt;coros/tasks&gt;)   <span class="hljs-comment"># Iter of coros. Each returns next result on await.</span>
</code></pre>
<div><h4 id="runsaterminalgamewhereyoucontrolanasteriskthatmustavoidnumbers">Runs a terminal game where you control an asterisk that must avoid numbers:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> asyncio, collections, curses, curses.textpad, enum, random

P = collections.namedtuple(<span class="hljs-string">'P'</span>, <span class="hljs-string">'x y'</span>)     <span class="hljs-comment"># Position (x and y coordinates).</span>
D = enum.Enum(<span class="hljs-string">'D'</span>, <span class="hljs-string">'n e s w'</span>)              <span class="hljs-comment"># Direction (north, east, etc.).</span>
W, H = <span class="hljs-number">15</span>, <span class="hljs-number">7</span>                               <span class="hljs-comment"># Width and height of the field.</span>

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main</span><span class="hljs-params">(screen)</span>:</span>
    curses.curs_set(<span class="hljs-number">0</span>)                     <span class="hljs-comment"># Makes the cursor invisible.</span>
    screen.nodelay(<span class="hljs-keyword">True</span>)                   <span class="hljs-comment"># Makes getch() non-blocking.</span>
    asyncio.run(main_coroutine(screen))    <span class="hljs-comment"># Starts running asyncio code.</span>

<span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main_coroutine</span><span class="hljs-params">(screen)</span>:</span>
    moves = asyncio.Queue()
    state = {<span class="hljs-string">'*'</span>: P(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>)} | {id_: P(W//<span class="hljs-number">2</span>, H//<span class="hljs-number">2</span>) <span class="hljs-keyword">for</span> id_ <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)}
    ai    = [random_controller(id_, moves) <span class="hljs-keyword">for</span> id_ <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)]
    mvc   = [controller(screen, moves), model(moves, state), view(state, screen)]
    tasks = [asyncio.create_task(coro) <span class="hljs-keyword">for</span> coro <span class="hljs-keyword">in</span> ai + mvc]
    <span class="hljs-keyword">await</span> asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)

<span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">random_controller</span><span class="hljs-params">(id_, moves)</span>:</span>
    <span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
        d = random.choice(list(D))
        moves.put_nowait((id_, d))
        <span class="hljs-keyword">await</span> asyncio.sleep(random.triangular(<span class="hljs-number">0.01</span>, <span class="hljs-number">0.65</span>))

<span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">controller</span><span class="hljs-params">(screen, moves)</span>:</span>
    <span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
        key_mappings = {<span class="hljs-number">258</span>: D.s, <span class="hljs-number">259</span>: D.n, <span class="hljs-number">260</span>: D.w, <span class="hljs-number">261</span>: D.e}
        <span class="hljs-keyword">if</span> d := key_mappings.get(screen.getch()):
            moves.put_nowait((<span class="hljs-string">'*'</span>, d))
        <span class="hljs-keyword">await</span> asyncio.sleep(<span class="hljs-number">0.005</span>)

<span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">model</span><span class="hljs-params">(moves, state)</span>:</span>
    <span class="hljs-keyword">while</span> state[<span class="hljs-string">'*'</span>] <span class="hljs-keyword">not</span> <span class="hljs-keyword">in</span> (state[id_] <span class="hljs-keyword">for</span> id_ <span class="hljs-keyword">in</span> range(<span class="hljs-number">10</span>)):
        id_, d = <span class="hljs-keyword">await</span> moves.get()
        deltas = {D.n: P(<span class="hljs-number">0</span>, <span class="hljs-number">-1</span>), D.e: P(<span class="hljs-number">1</span>, <span class="hljs-number">0</span>), D.s: P(<span class="hljs-number">0</span>, <span class="hljs-number">1</span>), D.w: P(<span class="hljs-number">-1</span>, <span class="hljs-number">0</span>)}
        state[id_] = P((state[id_].x + deltas[d].x) % W, (state[id_].y + deltas[d].y) % H)

<span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">view</span><span class="hljs-params">(state, screen)</span>:</span>
    offset = P(curses.COLS//<span class="hljs-number">2</span> - W//<span class="hljs-number">2</span>, curses.LINES//<span class="hljs-number">2</span> - H//<span class="hljs-number">2</span>)
    <span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
        screen.erase()
        curses.textpad.rectangle(screen, offset.y-<span class="hljs-number">1</span>, offset.x-<span class="hljs-number">1</span>, offset.y+H, offset.x+W)
        <span class="hljs-keyword">for</span> id_, p <span class="hljs-keyword">in</span> state.items():
            screen.addstr(offset.y + (p.y - state[<span class="hljs-string">'*'</span>].y + H//<span class="hljs-number">2</span>) % H,
                          offset.x + (p.x - state[<span class="hljs-string">'*'</span>].x + W//<span class="hljs-number">2</span>) % W, str(id_))
        screen.refresh()
        <span class="hljs-keyword">await</span> asyncio.sleep(<span class="hljs-number">0.005</span>)

<span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:
    curses.wrapper(main)
</code></pre></div>

<p><br></p>
<div><h1 id="libraries">Libraries</h1><div><h2 id="progressbar"><a href="#progressbar" name="progressbar">#</a>Progress Bar</h2><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install tqdm</span>
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">import</span> tqdm, time
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">for</span> el <span class="hljs-keyword">in</span> tqdm.tqdm([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>], desc=<span class="hljs-string">'Processing'</span>):
<span class="hljs-meta">... </span>    time.sleep(<span class="hljs-number">1</span>)
Processing: 100%|████████████████████| 3/3 [00:03&lt;00:00,  1.00s/it]
</code></pre></div></div>


<div><h2 id="plot"><a href="#plot" name="plot">#</a>Plot</h2><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install matplotlib</span>
<span class="hljs-keyword">import</span> matplotlib.pyplot <span class="hljs-keyword">as</span> plt

plt.plot/bar/scatter(x_data, y_data [, label=&lt;str&gt;])  <span class="hljs-comment"># Also plt.plot(y_data).</span>
plt.legend()                                          <span class="hljs-comment"># Adds a legend of labels.</span>
plt.title/xlabel/ylabel(&lt;str&gt;)                        <span class="hljs-comment"># Adds title or axis label.</span>
plt.show()                                            <span class="hljs-comment"># Also plt.savefig(&lt;path&gt;).</span>
plt.clf()                                             <span class="hljs-comment"># Clears the plot (figure).</span>
</code></pre></div>

<div><h2 id="table"><a href="#table" name="table">#</a>Table</h2><div><h4 id="printsacsvspreadsheettotheconsole">Prints a CSV spreadsheet to the console:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install tabulate</span>
<span class="hljs-keyword">import</span> csv, tabulate
<span class="hljs-keyword">with</span> open(<span class="hljs-string">'test.csv'</span>, encoding=<span class="hljs-string">'utf-8'</span>, newline=<span class="hljs-string">''</span>) <span class="hljs-keyword">as</span> file:
    rows = list(csv.reader(file))
print(tabulate.tabulate(rows, headers=<span class="hljs-string">'firstrow'</span>))
</code></pre></div></div>


<div><h2 id="consoleapp"><a href="#consoleapp" name="consoleapp">#</a>Console App</h2><div><h4 id="runsabasicfileexplorerintheconsole">Runs a basic file explorer in the console:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install windows-curses</span>
<span class="hljs-keyword">import</span> curses, os
<span class="hljs-keyword">from</span> curses <span class="hljs-keyword">import</span> A_REVERSE, KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main</span><span class="hljs-params">(screen)</span>:</span>
    ch, first, selected, paths = <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, os.listdir()
    <span class="hljs-keyword">while</span> ch != ord(<span class="hljs-string">'q'</span>):
        height, width = screen.getmaxyx()
        screen.erase()
        <span class="hljs-keyword">for</span> y, filename <span class="hljs-keyword">in</span> enumerate(paths[first : first+height]):
            color = A_REVERSE <span class="hljs-keyword">if</span> filename == paths[selected] <span class="hljs-keyword">else</span> <span class="hljs-number">0</span>
            screen.addnstr(y, <span class="hljs-number">0</span>, filename, width-<span class="hljs-number">1</span>, color)
        ch = screen.getch()
        selected -= (ch == KEY_UP) <span class="hljs-keyword">and</span> (selected &gt; <span class="hljs-number">0</span>)
        selected += (ch == KEY_DOWN) <span class="hljs-keyword">and</span> (selected &lt; len(paths)-<span class="hljs-number">1</span>)
        first -= (first &gt; selected)
        first += (first &lt; selected-(height-<span class="hljs-number">1</span>))
        <span class="hljs-keyword">if</span> ch <span class="hljs-keyword">in</span> [KEY_LEFT, KEY_RIGHT, ord(<span class="hljs-string">'\n'</span>)]:
            new_dir = <span class="hljs-string">'..'</span> <span class="hljs-keyword">if</span> ch == KEY_LEFT <span class="hljs-keyword">else</span> paths[selected]
            <span class="hljs-keyword">if</span> os.path.isdir(new_dir):
                os.chdir(new_dir)
                first, selected, paths = <span class="hljs-number">0</span>, <span class="hljs-number">0</span>, os.listdir()

<span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:
    curses.wrapper(main)
</code></pre></div></div>


<div><h2 id="guiapp"><a href="#guiapp" name="guiapp">#</a>GUI App</h2><div><h4 id="runsadesktopappforconvertingweightsfrommetricunitsintopounds">Runs a desktop app for converting weights from metric units into pounds:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install FreeSimpleGUI</span>
<span class="hljs-keyword">import</span> FreeSimpleGUI <span class="hljs-keyword">as</span> sg

text_box = sg.Input(default_text=<span class="hljs-string">'100'</span>, enable_events=<span class="hljs-keyword">True</span>, key=<span class="hljs-string">'QUANTITY'</span>)
dropdown = sg.InputCombo([<span class="hljs-string">'g'</span>, <span class="hljs-string">'kg'</span>, <span class="hljs-string">'t'</span>], <span class="hljs-string">'kg'</span>, readonly=<span class="hljs-keyword">True</span>, enable_events=<span class="hljs-keyword">True</span>, k=<span class="hljs-string">'UNIT'</span>)
label    = sg.Text(<span class="hljs-string">'100 kg is 220.462 lbs.'</span>, key=<span class="hljs-string">'OUTPUT'</span>)
window   = sg.Window(<span class="hljs-string">'Weight Converter'</span>, [[text_box, dropdown], [label], [sg.Button(<span class="hljs-string">'Close'</span>)]])

<span class="hljs-keyword">while</span> <span class="hljs-keyword">True</span>:
    event, values = window.read()
    <span class="hljs-keyword">if</span> event <span class="hljs-keyword">in</span> [sg.WIN_CLOSED, <span class="hljs-string">'Close'</span>]:
        <span class="hljs-keyword">break</span>
    <span class="hljs-keyword">try</span>:
        quantity = float(values[<span class="hljs-string">'QUANTITY'</span>])
    <span class="hljs-keyword">except</span> ValueError:
        <span class="hljs-keyword">continue</span>
    unit = values[<span class="hljs-string">'UNIT'</span>]
    lbs = quantity * {<span class="hljs-string">'g'</span>: <span class="hljs-number">0.001</span>, <span class="hljs-string">'kg'</span>: <span class="hljs-number">1</span>, <span class="hljs-string">'t'</span>: <span class="hljs-number">1000</span>}[unit] / <span class="hljs-number">0.45359237</span>
    window[<span class="hljs-string">'OUTPUT'</span>].update(value=<span class="hljs-string">f'<span class="hljs-subst">{quantity}</span> <span class="hljs-subst">{unit}</span> is <span class="hljs-subst">{lbs:g}</span> lbs.'</span>)
window.close()
</code></pre></div></div>


<div><h2 id="scraping"><a href="#scraping" name="scraping">#</a>Scraping</h2><div><h4 id="scrapespythonsurlandlogofromitswikipediapage">Scrapes Python's URL and logo from its Wikipedia page:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install requests beautifulsoup4</span>
<span class="hljs-keyword">import</span> requests, bs4, os

get = <span class="hljs-keyword">lambda</span> url: requests.get(url, headers={<span class="hljs-string">'User-Agent'</span>: <span class="hljs-string">'cpc-bot'</span>})
response = get(<span class="hljs-string">'https://en.wikipedia.org/wiki/Python_(programming_language)'</span>)
document = bs4.BeautifulSoup(response.text, <span class="hljs-string">'html.parser'</span>)
table = document.find(<span class="hljs-string">'table'</span>, class_=<span class="hljs-string">'infobox vevent'</span>)
python_url = table.find(<span class="hljs-string">'th'</span>, text=<span class="hljs-string">'Website'</span>).next_sibling.a[<span class="hljs-string">'href'</span>]
logo_url = table.find(<span class="hljs-string">'img'</span>)[<span class="hljs-string">'src'</span>]
filename = os.path.basename(logo_url)
<span class="hljs-keyword">with</span> open(filename, <span class="hljs-string">'wb'</span>) <span class="hljs-keyword">as</span> file:
    file.write(get(<span class="hljs-string">f'https:<span class="hljs-subst">{logo_url}</span>'</span>).content)
print(<span class="hljs-string">f'URL: <span class="hljs-subst">{python_url}</span>, logo: file://<span class="hljs-subst">{os.path.abspath(filename)}</span>'</span>)
</code></pre></div></div>


<div><h3 id="selenium">Selenium</h3><p><strong>Library for scraping websites with dynamic content.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install selenium</span>
<span class="hljs-keyword">from</span> selenium <span class="hljs-keyword">import</span> webdriver
</code></pre></div>


<pre><code class="python language-python hljs">&lt;Drv&gt; = webdriver.Chrome/Firefox/Safari/Edge()  <span class="hljs-comment"># Opens the browser. Also &lt;Driver&gt;.quit().</span>
&lt;Drv&gt;.implicitly_wait(seconds)                  <span class="hljs-comment"># Sets timeout for find_element/s() methods.</span>
&lt;Drv&gt;.get(<span class="hljs-string">'&lt;url&gt;'</span>)                              <span class="hljs-comment"># Blocks until browser fires the load event.</span>
&lt;str&gt; = &lt;Drv&gt;.page_source                       <span class="hljs-comment"># Returns HTML of the page's current state.</span>
&lt;El&gt;  = &lt;Drv/El&gt;.find_element(<span class="hljs-string">'xpath'</span>, &lt;str&gt;)   <span class="hljs-comment"># Accepts '//&lt;tag&gt;[@&lt;attr_name&gt;="&lt;val&gt;"]…'.</span>
&lt;str&gt; = &lt;El&gt;.get_attribute(<span class="hljs-string">'&lt;name&gt;'</span>)            <span class="hljs-comment"># Returns attribute or property if exists.</span>
&lt;El&gt;.click/clear()                              <span class="hljs-comment"># Also &lt;El&gt;.text and &lt;El&gt;.send_keys(&lt;str&gt;).</span>
</code></pre>
<div><h4 id="xpathalsoavailableinlxmlscrapyandbrowsersconsoleviadxxpath">XPath — also available in lxml, Scrapy, and browser's console via <code class="python hljs"><span class="hljs-string">'$x("&lt;xpath&gt;")'</span></code>:</h4><pre><code class="python language-python hljs">&lt;xpath&gt;     = //&lt;element&gt;[/ <span class="hljs-keyword">or</span> // &lt;element&gt;]    <span class="hljs-comment"># E.g. …/child, …//descendant, …/../sibling.</span>
&lt;xpath&gt;     = //&lt;element&gt;/following::&lt;element&gt;  <span class="hljs-comment"># Next element. Also preceding::, parent::.</span>
&lt;element&gt;   = &lt;tag&gt;&lt;conditions&gt;&lt;index&gt;          <span class="hljs-comment"># Tag accepts */a/…. Use [1/2/…] for index.</span>
&lt;condition&gt; = [&lt;sub_cond&gt; [<span class="hljs-keyword">and</span>/<span class="hljs-keyword">or</span> &lt;sub_cond&gt;]]  <span class="hljs-comment"># Use not(&lt;sub_cond&gt;) to negate condition.</span>
&lt;sub_cond&gt;  = @&lt;attr&gt;[=<span class="hljs-string">"&lt;val&gt;"</span>]                 <span class="hljs-comment"># `text()=` and `.=` match (complete) text.</span>
&lt;sub_cond&gt;  = contains(@&lt;attr&gt;, <span class="hljs-string">"&lt;val&gt;"</span>)        <span class="hljs-comment"># Is &lt;val&gt; a substring of attribute's value?</span>
&lt;sub_cond&gt;  = [//]&lt;element&gt;                     <span class="hljs-comment"># Has matching child? Descendant if //&lt;el&gt;.</span>
</code></pre></div>

<div><h2 id="webapp"><a href="#webapp" name="webapp">#</a>Web App</h2><p><strong>Flask is a micro web framework/server. If you just want to open a html file in a web browser use <code class="python hljs"><span class="hljs-string">'webbrowser.open(&lt;path&gt;)'</span></code> instead.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install flask</span>
<span class="hljs-keyword">import</span> flask <span class="hljs-keyword">as</span> fl
</code></pre></div>


<pre><code class="python language-python hljs">app = fl.Flask(__name__)                   <span class="hljs-comment"># Returns the app object. Put at the top.</span>
app.run(host=<span class="hljs-keyword">None</span>, port=<span class="hljs-keyword">None</span>, debug=<span class="hljs-keyword">None</span>)  <span class="hljs-comment"># Same as `$ flask --app FILE run --ARG=VAL`.</span>
</code></pre>
<ul>
<li><strong>Starts the app at <code class="python hljs"><span class="hljs-string">'http://localhost:5000'</span></code>. Use <code class="python hljs"><span class="hljs-string">'host="0.0.0.0"'</span></code> to run externally.</strong></li>
<li><strong>Install a WSGI server like <a href="https://flask.palletsprojects.com/en/latest/deploying/waitress/">Waitress</a> and a HTTP server such as <a href="https://flask.palletsprojects.com/en/latest/deploying/nginx/">Nginx</a> for better security.</strong></li>
<li><strong>Debug mode restarts the app whenever script changes and displays errors in the browser.</strong></li>
</ul>
<div><h3 id="servingfiles">Serving Files</h3><pre><code class="python language-python hljs"><span class="hljs-meta">@app.route('/img/&lt;path:filename&gt;')</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">serve_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">return</span> fl.send_from_directory(<span class="hljs-string">'DIRNAME'</span>, filename)
</code></pre></div>

<div><h3 id="servinghtml">Serving HTML</h3><pre><code class="python language-python hljs"><span class="hljs-meta">@app.route('/&lt;sport&gt;')</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">serve_html</span><span class="hljs-params">(sport)</span>:</span>
    <span class="hljs-keyword">return</span> fl.render_template_string(<span class="hljs-string">'&lt;h1&gt;{{title}}&lt;/h1&gt;'</span>, title=sport)
</code></pre></div>

<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'fl.render_template(filename, &lt;kwargs&gt;)'</span></code> renders a file located in 'templates' dir.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fl.abort(&lt;int&gt;)'</span></code> returns error code and <code class="python hljs"><span class="hljs-string">'return fl.redirect(&lt;url&gt;)'</span></code> redirects.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fl.request.args[&lt;str&gt;]'</span></code> returns parameter from query string (URL part right of '?').</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'fl.session[&lt;str&gt;] = &lt;obj&gt;'</span></code> stores session data. It requires secret key to be set at the startup with <code class="python hljs"><span class="hljs-string">'app.secret_key = &lt;str&gt;'</span></code>.</strong></li>
</ul>
<div><h3 id="servingjson">Serving JSON</h3><pre><code class="python language-python hljs"><span class="hljs-meta">@app.post('/&lt;sport&gt;/odds')</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">serve_json</span><span class="hljs-params">(sport)</span>:</span>
    team = fl.request.form[<span class="hljs-string">'team'</span>]
    <span class="hljs-keyword">return</span> {<span class="hljs-string">'team'</span>: team, <span class="hljs-string">'odds'</span>: [<span class="hljs-number">2.09</span>, <span class="hljs-number">3.74</span>, <span class="hljs-number">3.68</span>]}
</code></pre></div>

<div><h4 id="startstheappinitsownthreadandqueriesitsrestapi">Starts the app in its own thread and queries its REST API:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install requests</span>
<span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">import</span> threading, requests
<span class="hljs-meta">&gt;&gt;&gt; </span>threading.Thread(target=app.run, daemon=<span class="hljs-keyword">True</span>).start()
<span class="hljs-meta">&gt;&gt;&gt; </span>url = <span class="hljs-string">'http://localhost:5000/football/odds'</span>
<span class="hljs-meta">&gt;&gt;&gt; </span>response = requests.post(url, data={<span class="hljs-string">'team'</span>: <span class="hljs-string">'arsenal f.c.'</span>})
<span class="hljs-meta">&gt;&gt;&gt; </span>response.json()
{<span class="hljs-string">'team'</span>: <span class="hljs-string">'arsenal f.c.'</span>, <span class="hljs-string">'odds'</span>: [<span class="hljs-number">2.09</span>, <span class="hljs-number">3.74</span>, <span class="hljs-number">3.68</span>]}
</code></pre></div>

<div><h2 id="profiling"><a href="#profiling" name="profiling">#</a>Profiling</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> time <span class="hljs-keyword">import</span> perf_counter
start_time = perf_counter()
...
duration_in_seconds = perf_counter() - start_time
</code></pre></div>

<div><h3 id="timingasnippet">Timing a Snippet</h3><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span><span class="hljs-keyword">from</span> timeit <span class="hljs-keyword">import</span> timeit
<span class="hljs-meta">&gt;&gt;&gt; </span>timeit(<span class="hljs-string">'list(range(10000))'</span>, number=<span class="hljs-number">1000</span>, globals=globals(), setup=<span class="hljs-string">'pass'</span>)
<span class="hljs-number">0.19373</span>
</code></pre></div>

<div><h3 id="profilingbyline">Profiling by Line</h3><pre><code class="text language-text">$ pip3 install line_profiler
$ echo '@profile
def main():
    a = list(range(10000))
    b = set(range(10000))
main()' &gt; test.py
$ kernprof -lv test.py
Line #      Hits         Time  Per Hit   % Time  Line Contents
==============================================================
     1                                           @profile
     2                                           def main():
     3         1        253.4    253.4     32.2      a = list(range(10000))
     4         1        534.1    534.1     67.8      b = set(range(10000))
</code></pre></div>

<div><h3 id="callandflamegraphs">Call and Flame Graphs</h3><pre><code class="bash language-bash hljs">$ apt/brew install graphviz &amp;&amp; pip3 install gprof2dot snakeviz  <span class="hljs-comment"># Or download installer.</span>
$ tail --lines=+2 test.py &gt; test.py                             <span class="hljs-comment"># Removes the first line.</span>
$ python3 -m cProfile -o test.prof test.py                      <span class="hljs-comment"># Runs a tracing profiler.</span>
$ gprof2dot --format=pstats test.prof | dot -T png -o test.png  <span class="hljs-comment"># Generates a call graph.</span>
$ xdg-open/open test.png                                        <span class="hljs-comment"># Displays the call graph.</span>
$ snakeviz test.prof                                            <span class="hljs-comment"># Displays a flame graph.</span>
</code></pre></div>

<div><h3 id="samplingandmemoryprofilers">Sampling and Memory Profilers</h3><pre><code class="text language-text">┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━┯━━━━━━┓
┃ pip3 install │   Target   │          How to run           │ Lines │ Live ┃
┠──────────────┼────────────┼───────────────────────────────┼───────┼──────┨
┃ pyinstrument │    CPU     │ pyinstrument test.py          │   ×   │  ×   ┃
┃ py-spy       │    CPU     │ py-spy top -- python3 test.py │   ×   │  ✓   ┃
┃ scalene      │ CPU+Memory │ scalene test.py               │   ✓   │  ×   ┃
┃ memray       │   Memory   │ memray run --live test.py     │   ✓   │  ✓   ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━┷━━━━━━┛
</code></pre></div>

<div><h2 id="numpy"><a href="#numpy" name="numpy">#</a>NumPy</h2><p><strong>Array manipulation mini-language. It can run up to one hundred times faster than the equivalent Python code. An even faster alternative that runs on a GPU is called CuPy.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install numpy</span>
<span class="hljs-keyword">import</span> numpy <span class="hljs-keyword">as</span> np
</code></pre></div>


<pre><code class="python language-python hljs">&lt;array&gt; = np.array(&lt;list/list_of_lists/…&gt;)            <span class="hljs-comment"># NumPy array of one or more dimensions.</span>
&lt;array&gt; = np.zeros/ones/empty(shape)                  <span class="hljs-comment"># Pass a tuple of ints (dimension sizes).</span>
&lt;array&gt; = np.arange(from_inc, to_exc, ±step)          <span class="hljs-comment"># Also np.linspace(start, stop, length).</span>
&lt;array&gt; = np.random.randint(from_inc, to_exc, shape)  <span class="hljs-comment"># Also random.uniform(low, high, shape).</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;view&gt;  = &lt;array&gt;.reshape(shape)                      <span class="hljs-comment"># Also `&lt;array&gt;.shape = (&lt;int&gt;, [...])`.</span>
&lt;array&gt; = &lt;array&gt;.flatten()                           <span class="hljs-comment"># Returns 1d copy. Also &lt;array&gt;.ravel().</span>
&lt;view&gt;  = &lt;array&gt;.transpose()                         <span class="hljs-comment"># Flips the table over its main diagonal.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;array&gt; = np.copy/abs/sqrt/log/int64(&lt;array&gt;)         <span class="hljs-comment"># Returns a new array of the same shape.</span>
&lt;array&gt; = &lt;array&gt;.sum/max/mean/argmax/all(axis)       <span class="hljs-comment"># Aggregates dimension with passed index.</span>
&lt;array&gt; = np.apply_along_axis(&lt;func&gt;, axis, &lt;array&gt;)  <span class="hljs-comment"># Func can return a scalar or an array.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;array&gt; = np.concatenate(&lt;list_of_arrays&gt;, axis=<span class="hljs-number">0</span>)    <span class="hljs-comment"># Links arrays along first axis (rows).</span>
&lt;array&gt; = np.vstack/column_stack(&lt;list_of_arrays&gt;)    <span class="hljs-comment"># A 1d array is treated as a row/column.</span>
&lt;array&gt; = np.tile/repeat(&lt;array&gt;, &lt;int/s&gt; [, axis])   <span class="hljs-comment"># Tiles the array or repeats elements.</span>
</code></pre>
<ul>
<li><strong>Shape is a tuple of dimension sizes. A 100x50 RGB image has shape (50, 100, 3).</strong></li>
<li><strong>Axis is an index of a dimension. Leftmost dimension has index 0. Summing the RGB image along axis 2 will return a greyscale image with shape (50, 100).</strong></li>
</ul>
<div><h3 id="indexing">Indexing</h3><pre><code class="bash hljs">&lt;el&gt;       = &lt;2d&gt;[row_index, col_index]               <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int&gt;, &lt;int&gt;, &lt;int&gt;]`.</span>
&lt;1d_view&gt;  = &lt;2d&gt;[row_index]                          <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int&gt;, &lt;int&gt;, &lt;slice&gt;]`.</span>
&lt;1d_view&gt;  = &lt;2d&gt;[:, col_index]                       <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int&gt;, &lt;slice&gt;, &lt;int&gt;]`.</span>
&lt;2d_view&gt;  = &lt;2d&gt;[from:to_row_i, from:to_col_i]       <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int&gt;, &lt;slice&gt;, &lt;slice&gt;]`.</span>
</code></pre></div>

<pre><code class="bash hljs">&lt;1d_array&gt; = &lt;2d&gt;[row_indices, col_indices]           <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int/1d&gt;, &lt;1d&gt;, &lt;1d&gt;]`.</span>
&lt;2d_array&gt; = &lt;2d&gt;[row_indices]                        <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int/1d&gt;, &lt;1d&gt;, &lt;slice&gt;]`.</span>
&lt;2d_array&gt; = &lt;2d&gt;[:, col_indices]                     <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int/1d&gt;, &lt;slice&gt;, &lt;1d&gt;]`.</span>
&lt;2d_array&gt; = &lt;2d&gt;[np.ix_(row_indices, col_indices)]   <span class="hljs-comment"># Also `&lt;3d&gt;[&lt;int/1d/2d&gt;, &lt;2d&gt;, &lt;2d&gt;]`.</span>
</code></pre>
<pre><code class="bash hljs">&lt;2d_bools&gt; = &lt;2d&gt; &gt; &lt;el/1d/2d&gt;                        <span class="hljs-comment"># A 1d object must be a size of a row.</span>
&lt;1/2d_arr&gt; = &lt;2d&gt;[&lt;2d/1d_bools&gt;]                      <span class="hljs-comment"># A 1d object must be a size of a col.</span>
</code></pre>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">':'</span></code> returns a slice of all dimension's indices. Omitted dimensions default to <code class="python hljs"><span class="hljs-string">':'</span></code>.</strong></li>
<li><strong>Python converts <code class="python hljs"><span class="hljs-string">'obj[i, j]'</span></code> to <code class="python hljs"><span class="hljs-string">'obj[(i, j)]'</span></code>. This makes <code class="python hljs"><span class="hljs-string">'&lt;2d&gt;[row_i, col_i]'</span></code> and <code class="python hljs"><span class="hljs-string">'&lt;2d&gt;[row_indices]'</span></code> indistinguishable to NumPy if tuple of two indices is passed!</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'ix_([1, 2], [3, 4])'</span></code> returns <code class="python hljs"><span class="hljs-string">'[[1], [2]]'</span></code> and <code class="python hljs"><span class="hljs-string">'[[3, 4]]'</span></code>. Due to broadcasting rules, this is the same as using <code class="python hljs"><span class="hljs-string">'[[1, 1], [2, 2]]'</span></code> and <code class="python hljs"><span class="hljs-string">'[[3, 4], [3, 4]]'</span></code>.</strong></li>
<li><strong>Any value that is broadcastable to the indexed shape can be assigned to the selection.</strong></li>
</ul>
<div><h3 id="broadcasting">Broadcasting</h3><p><strong>A set of rules by which NumPy functions operate on arrays of different shapes.</strong></p><pre><code class="python language-python hljs">left  = np.array([<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.8</span>])                   <span class="hljs-comment"># I.e. `left.shape  == (3,)`.</span>
right = np.array([[<span class="hljs-number">0.1</span>], [<span class="hljs-number">0.6</span>], [<span class="hljs-number">0.8</span>]])               <span class="hljs-comment"># I.e. `right.shape == (3, 1)`.</span>
</code></pre></div>


<div><h4 id="1ifarrayshapesdifferinlengthleftpadtheshortershapewithones">1. If array shapes differ in length, left-pad the shorter shape with ones:</h4><pre><code class="python language-python hljs">left  = np.array([[<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.8</span>]])                 <span class="hljs-comment"># I.e. `left.shape  == (1, 3)`.</span>
right = np.array([[<span class="hljs-number">0.1</span>], [<span class="hljs-number">0.6</span>], [<span class="hljs-number">0.8</span>]])               <span class="hljs-comment"># I.e. `right.shape == (3, 1)`.</span>
</code></pre></div>

<div><h4 id="2ifanydimensionsdifferinsizeexpandtheonesthathavesize1byduplicatingtheirelements">2. If any dimensions differ in size, expand the ones that have size 1 by duplicating their elements:</h4><pre><code class="python language-python hljs">left  = np.array([[<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.8</span>],                  <span class="hljs-comment"># I.e. `left.shape  == (3, 3)`.</span>
                  [<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.8</span>],
                  [<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.8</span>]])

right = np.array([[<span class="hljs-number">0.1</span>,  <span class="hljs-number">0.1</span>,  <span class="hljs-number">0.1</span>],                  <span class="hljs-comment"># I.e. `right.shape == (3, 3)`.</span>
                  [<span class="hljs-number">0.6</span>,  <span class="hljs-number">0.6</span>,  <span class="hljs-number">0.6</span>],
                  [<span class="hljs-number">0.8</span>,  <span class="hljs-number">0.8</span>,  <span class="hljs-number">0.8</span>]])
</code></pre></div>

<div><h3 id="example-3">Example</h3><div><h4 id="foreachpointreturnsindexofitsnearestpoint010608121">For each point returns index of its nearest point (<code class="python hljs">[<span class="hljs-number">0.1</span>, <span class="hljs-number">0.6</span>, <span class="hljs-number">0.8</span>] =&gt; [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">1</span>]</code>):</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>print(points := np.array([<span class="hljs-number">0.1</span>, <span class="hljs-number">0.6</span>, <span class="hljs-number">0.8</span>]))
[<span class="hljs-number">0.1</span>  <span class="hljs-number">0.6</span>  <span class="hljs-number">0.8</span>]
<span class="hljs-meta">&gt;&gt;&gt; </span>print(wrapped_points := points.reshape(<span class="hljs-number">3</span>, <span class="hljs-number">1</span>))
[[<span class="hljs-number">0.1</span>]
 [<span class="hljs-number">0.6</span>]
 [<span class="hljs-number">0.8</span>]]
<span class="hljs-meta">&gt;&gt;&gt; </span>print(deltas := points - wrapped_points)
[[ <span class="hljs-number">0.</span>   <span class="hljs-number">0.5</span>  <span class="hljs-number">0.7</span>]
 [<span class="hljs-number">-0.5</span>  <span class="hljs-number">0.</span>   <span class="hljs-number">0.2</span>]
 [<span class="hljs-number">-0.7</span> <span class="hljs-number">-0.2</span>  <span class="hljs-number">0.</span> ]]
<span class="hljs-meta">&gt;&gt;&gt; </span>deltas[range(<span class="hljs-number">3</span>), range(<span class="hljs-number">3</span>)] = np.inf
<span class="hljs-meta">&gt;&gt;&gt; </span>print(distances := np.abs(deltas))
[[inf  <span class="hljs-number">0.5</span>  <span class="hljs-number">0.7</span>]
 [<span class="hljs-number">0.5</span>  inf  <span class="hljs-number">0.2</span>]
 [<span class="hljs-number">0.7</span>  <span class="hljs-number">0.2</span>  inf]]
<span class="hljs-meta">&gt;&gt;&gt; </span>print(distances.argmin(axis=<span class="hljs-number">1</span>))
[<span class="hljs-number">1</span> <span class="hljs-number">2</span> <span class="hljs-number">1</span>]
</code></pre></div></div>


<div><h2 id="image"><a href="#image" name="image">#</a>Image</h2><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install pillow</span>
<span class="hljs-keyword">from</span> PIL <span class="hljs-keyword">import</span> Image
</code></pre></div>

<pre><code class="python language-python hljs">&lt;Image&gt; = Image.new(<span class="hljs-string">'RGB'</span>, (width, height))   <span class="hljs-comment"># Creates an image. Also `color=&lt;tuple_of_ints&gt;`.</span>
&lt;Image&gt; = Image.open(&lt;path&gt;)                  <span class="hljs-comment"># Identifies format based on the file's contents.</span>
&lt;Image&gt; = &lt;Image&gt;.convert(<span class="hljs-string">'&lt;mode&gt;'</span>)           <span class="hljs-comment"># Converts the image to the new mode (see Modes).</span>
&lt;Image&gt;.save(&lt;path&gt;)                          <span class="hljs-comment"># Also `quality=&lt;int&gt;` if extension is jpg/jpeg.</span>
&lt;Image&gt;.show()                                <span class="hljs-comment"># Displays image in system's default preview app.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;int/tup&gt; = &lt;Image&gt;.getpixel((x, y))          <span class="hljs-comment"># Returns the pixel's value, that is, its color.</span>
&lt;ImgCore&gt; = &lt;Image&gt;.getdata()                 <span class="hljs-comment"># Returns a flattened view of the pixel values.</span>
&lt;Image&gt;.putpixel((x, y), &lt;int/tuple&gt;)         <span class="hljs-comment"># Updates pixel's value. Clips passed integer/s.</span>
&lt;Image&gt;.putdata(&lt;list/ImgCore&gt;)               <span class="hljs-comment"># Updates pixels with a copy of passed sequence.</span>
&lt;Image&gt;.paste(&lt;Image&gt;, (x, y))                <span class="hljs-comment"># Draws passed image at the specified location.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Image&gt; = &lt;Image&gt;.filter(&lt;Filter&gt;)            <span class="hljs-comment"># E.g. `&lt;Image&gt;.filter(ImageFilter.FIND_EDGES)`.</span>
&lt;Image&gt; = &lt;Enhance&gt;.enhance(&lt;float&gt;)          <span class="hljs-comment"># E.g. `ImageEnhance.Color(&lt;Image&gt;).enhance(2)`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;array&gt; = np.array(&lt;Image&gt;)                   <span class="hljs-comment"># Creates a 2d or 3d NumPy array from the image.</span>
&lt;Image&gt; = Image.fromarray(np.uint8(&lt;array&gt;))  <span class="hljs-comment"># Use `&lt;array&gt;.clip(0, 255)` to clip the values.</span>
</code></pre>
<div><h3 id="modes-1">Modes</h3><ul>
<li><strong><code class="python hljs"><span class="hljs-string">'L'</span></code> - Lightness (greyscale image). Each pixel is an integer between 0 and 255.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'RGB'</span></code> - Red, green, blue (true color image). Each pixel is a tuple of three integers.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'RGBA'</span></code> - RGB with alpha. Low alpha (i.e. fourth int) makes pixel more transparent.</strong></li>
<li><strong><code class="python hljs"><span class="hljs-string">'HSV'</span></code> - Hue, saturation, value. Three ints representing color in HSV color space.</strong></li>
</ul><div><h3 id="examples">Examples</h3><div><h4 id="createsapngimageofarainbowgradient">Creates a PNG image of a rainbow gradient:</h4><pre><code class="python language-python hljs">WIDTH, HEIGHT = <span class="hljs-number">100</span>, <span class="hljs-number">100</span>
n_pixels = WIDTH * HEIGHT
hues = (<span class="hljs-number">255</span> * i/n_pixels <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(n_pixels))
img = Image.new(<span class="hljs-string">'HSV'</span>, (WIDTH, HEIGHT))
img.putdata([(int(h), <span class="hljs-number">255</span>, <span class="hljs-number">255</span>) <span class="hljs-keyword">for</span> h <span class="hljs-keyword">in</span> hues])
img.convert(<span class="hljs-string">'RGB'</span>).save(<span class="hljs-string">'test.png'</span>)
</code></pre></div></div></div>




<div><h4 id="addsnoisetothepngimageanddisplaysit">Adds noise to the PNG image and displays it:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> randint
add_noise = <span class="hljs-keyword">lambda</span> value: max(<span class="hljs-number">0</span>, min(<span class="hljs-number">255</span>, value + randint(<span class="hljs-number">-20</span>, <span class="hljs-number">20</span>)))
img = Image.open(<span class="hljs-string">'test.png'</span>).convert(<span class="hljs-string">'HSV'</span>)
img.putdata([(add_noise(h), s, v) <span class="hljs-keyword">for</span> h, s, v <span class="hljs-keyword">in</span> img.getdata()])
img.show()
</code></pre></div>

<div><h3 id="imagedraw">Image Draw</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> PIL <span class="hljs-keyword">import</span> ImageDraw
&lt;Draw&gt; = ImageDraw.Draw(&lt;Image&gt;)              <span class="hljs-comment"># An object for adding 2D graphics to the image.</span>
&lt;Draw&gt;.point((x, y))                          <span class="hljs-comment"># Draws a point. Accepts `fill=&lt;int/tuple/str&gt;`.</span>
&lt;Draw&gt;.line((x1, y1, x2, y2 [, ...]))         <span class="hljs-comment"># For anti-aliasing use &lt;Image&gt;.resize((w, h)).</span>
&lt;Draw&gt;.arc((x1, y1, x2, y2), deg1, deg2)      <span class="hljs-comment"># Draws ellipse's arc in a clockwise direction.</span>
&lt;Draw&gt;.rectangle((x1, y1, x2, y2))            <span class="hljs-comment"># Also rounded_rectangle() and regular_polygon().</span>
&lt;Draw&gt;.polygon((x1, y1, x2, y2, ...))         <span class="hljs-comment"># The last point gets connected to the first one.</span>
&lt;Draw&gt;.ellipse((x1, y1, x2, y2))              <span class="hljs-comment"># To rotate it use &lt;Image&gt;.rotate(anticlock_deg).</span>
&lt;Draw&gt;.text((x, y), &lt;str&gt;)                    <span class="hljs-comment"># Also `font=ImageFont.truetype(&lt;path&gt;, size)`.</span>
</code></pre></div>

<ul>
<li><strong>Pass <code class="python hljs"><span class="hljs-string">'fill=&lt;color&gt;'</span></code> to set the figure's primary color.</strong></li>
<li><strong>Pass <code class="python hljs"><span class="hljs-string">'width=&lt;int&gt;'</span></code> to set the width of lines or contours.</strong></li>
<li><strong>Pass <code class="python hljs"><span class="hljs-string">'outline=&lt;color&gt;'</span></code> to set the color of the contours.</strong></li>
<li><strong>Color can be an int, tuple, <code class="python hljs"><span class="hljs-string">'#rrggbb[aa]'</span></code> or a color name.</strong></li>
</ul>
<div><h2 id="animation"><a href="#animation" name="animation">#</a>Animation</h2><div><h4 id="createsagifofabouncingball">Creates a GIF of a bouncing ball:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install imageio</span>
<span class="hljs-keyword">from</span> PIL <span class="hljs-keyword">import</span> Image, ImageDraw
<span class="hljs-keyword">import</span> imageio

WIDTH, HEIGHT, R = <span class="hljs-number">126</span>, <span class="hljs-number">126</span>, <span class="hljs-number">10</span>
frames = []
<span class="hljs-keyword">for</span> velocity <span class="hljs-keyword">in</span> range(<span class="hljs-number">1</span>, <span class="hljs-number">16</span>):
    y = sum(range(velocity))
    frame = Image.new(<span class="hljs-string">'L'</span>, (WIDTH, HEIGHT))
    draw = ImageDraw.Draw(frame)
    draw.ellipse((WIDTH/<span class="hljs-number">2</span>-R, y, WIDTH/<span class="hljs-number">2</span>+R, y+R*<span class="hljs-number">2</span>), fill=<span class="hljs-string">'white'</span>)
    frames.append(frame)
frames += reversed(frames[<span class="hljs-number">1</span>:<span class="hljs-number">-1</span>])
imageio.mimsave(<span class="hljs-string">'test.gif'</span>, frames, duration=<span class="hljs-number">0.03</span>)
</code></pre></div></div>


<div><h2 id="audio"><a href="#audio" name="audio">#</a>Audio</h2><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> wave
</code></pre></div>

<pre><code class="python language-python hljs">&lt;Wave&gt;  = wave.open(<span class="hljs-string">'&lt;path&gt;'</span>)         <span class="hljs-comment"># Opens specified WAV file for reading.</span>
&lt;int&gt;   = &lt;Wave&gt;.getframerate()       <span class="hljs-comment"># Returns number of frames per second.</span>
&lt;int&gt;   = &lt;Wave&gt;.getnchannels()       <span class="hljs-comment"># Returns number of samples per frame.</span>
&lt;int&gt;   = &lt;Wave&gt;.getsampwidth()       <span class="hljs-comment"># Returns number of bytes per sample.</span>
&lt;tuple&gt; = &lt;Wave&gt;.getparams()          <span class="hljs-comment"># Returns namedtuple of all parameters.</span>
&lt;bytes&gt; = &lt;Wave&gt;.readframes(nframes)  <span class="hljs-comment"># Returns all frames if `-1` is passed.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Wave&gt; = wave.open(<span class="hljs-string">'&lt;path&gt;'</span>, <span class="hljs-string">'wb'</span>)    <span class="hljs-comment"># Creates/truncates a file for writing.</span>
&lt;Wave&gt;.setframerate(&lt;int&gt;)            <span class="hljs-comment"># Pass 44100, or 48000 for video track.</span>
&lt;Wave&gt;.setnchannels(&lt;int&gt;)            <span class="hljs-comment"># Pass 1 for mono, 2 for stereo signal.</span>
&lt;Wave&gt;.setsampwidth(&lt;int&gt;)            <span class="hljs-comment"># Pass 2 for CD, 3 for hi-res quality.</span>
&lt;Wave&gt;.setparams(&lt;tuple&gt;)             <span class="hljs-comment"># Passed tuple must contain all params.</span>
&lt;Wave&gt;.writeframes(&lt;bytes&gt;)           <span class="hljs-comment"># Appends passed frames to audio file.</span>
</code></pre>
<ul>
<li><strong>Bytes object contains a sequence of frames, each consisting of one or more samples.</strong></li>
<li><strong>In a stereo signal, the first sample of a frame belongs to the left channel.</strong></li>
<li><strong>Each sample consists of one or more bytes that, when converted to an integer, indicate the displacement of a speaker membrane at a given moment.</strong></li>
<li><strong>If sample width is one byte, then the integer should be encoded unsigned. For all other sizes, the integer should be encoded signed with little-endian byte order.</strong></li>
</ul>
<div><h3 id="samplevalues">Sample Values</h3><pre><code class="python hljs">┏━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━┯━━━━━━━━━━━┓
┃ sampwidth │    min    │ zero │    max    ┃
┠───────────┼───────────┼──────┼───────────┨
┃     <span class="hljs-number">1</span>     │         <span class="hljs-number">0</span> │  <span class="hljs-number">128</span> │       <span class="hljs-number">255</span> ┃
┃     <span class="hljs-number">2</span>     │    <span class="hljs-number">-32768</span> │    <span class="hljs-number">0</span> │     <span class="hljs-number">32767</span> ┃
┃     <span class="hljs-number">3</span>     │  <span class="hljs-number">-8388608</span> │    <span class="hljs-number">0</span> │   <span class="hljs-number">8388607</span> ┃
┗━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━┷━━━━━━━━━━━┛
</code></pre></div>

<div><h3 id="readfloatsamplesfromwavfile">Read Float Samples from WAV File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_wav_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_int</span><span class="hljs-params">(bytes_obj)</span>:</span>
        an_int = int.from_bytes(bytes_obj, <span class="hljs-string">'little'</span>, signed=(p.sampwidth != <span class="hljs-number">1</span>))
        <span class="hljs-keyword">return</span> an_int - <span class="hljs-number">128</span> * (p.sampwidth == <span class="hljs-number">1</span>)
    <span class="hljs-keyword">with</span> wave.open(filename) <span class="hljs-keyword">as</span> file:
        p = file.getparams()
        frames = file.readframes(<span class="hljs-number">-1</span>)
    bytes_samples = (frames[i : i + p.sampwidth] <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">0</span>, len(frames), p.sampwidth))
    <span class="hljs-keyword">return</span> [get_int(b) / pow(<span class="hljs-number">2</span>, (p.sampwidth * <span class="hljs-number">8</span>) - <span class="hljs-number">1</span>) <span class="hljs-keyword">for</span> b <span class="hljs-keyword">in</span> bytes_samples], p
</code></pre></div>

<div><h3 id="writefloatsamplestowavfile">Write Float Samples to WAV File</h3><pre><code class="python language-python hljs"><span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">write_to_wav_file</span><span class="hljs-params">(filename, samples_f, p=<span class="hljs-keyword">None</span>, nchannels=<span class="hljs-number">1</span>, sampwidth=<span class="hljs-number">2</span>, framerate=<span class="hljs-number">44100</span>)</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_bytes</span><span class="hljs-params">(a_float)</span>:</span>
        a_float = max(<span class="hljs-number">-1</span>, min(<span class="hljs-number">1</span> - <span class="hljs-number">2e-16</span>, a_float))
        a_float += (p.sampwidth == <span class="hljs-number">1</span>)
        a_float *= pow(<span class="hljs-number">2</span>, (p.sampwidth * <span class="hljs-number">8</span>) - <span class="hljs-number">1</span>)
        <span class="hljs-keyword">return</span> int(a_float).to_bytes(p.sampwidth, <span class="hljs-string">'little'</span>, signed=(p.sampwidth != <span class="hljs-number">1</span>))
    <span class="hljs-keyword">if</span> p <span class="hljs-keyword">is</span> <span class="hljs-keyword">None</span>:
        p = wave._wave_params(nchannels, sampwidth, framerate, <span class="hljs-number">0</span>, <span class="hljs-string">'NONE'</span>, <span class="hljs-string">'not compressed'</span>)
    <span class="hljs-keyword">with</span> wave.open(filename, <span class="hljs-string">'wb'</span>) <span class="hljs-keyword">as</span> file:
        file.setparams(p)
        file.writeframes(<span class="hljs-string">b''</span>.join(get_bytes(f) <span class="hljs-keyword">for</span> f <span class="hljs-keyword">in</span> samples_f))
</code></pre></div>

<div><h3 id="examples-1">Examples</h3><div><h4 id="savesa440hzsinewavetoamonowavfile">Saves a 440 Hz sine wave to a mono WAV file:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> math <span class="hljs-keyword">import</span> pi, sin
samples_f = (sin(i * <span class="hljs-number">2</span> * pi * <span class="hljs-number">440</span> / <span class="hljs-number">44100</span>) <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(<span class="hljs-number">100_000</span>))
write_to_wav_file(<span class="hljs-string">'test.wav'</span>, samples_f)
</code></pre></div></div>


<div><h4 id="addsnoisetothewavfile">Adds noise to the WAV file:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> uniform
samples_f, params = read_wav_file(<span class="hljs-string">'test.wav'</span>)
samples_f = (f + uniform(<span class="hljs-number">-0.05</span>, <span class="hljs-number">0.05</span>) <span class="hljs-keyword">for</span> f <span class="hljs-keyword">in</span> samples_f)
write_to_wav_file(<span class="hljs-string">'test.wav'</span>, samples_f, p=params)
</code></pre></div>

<div><h4 id="playsthewavfile">Plays the WAV file:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install simpleaudio</span>
<span class="hljs-keyword">from</span> simpleaudio <span class="hljs-keyword">import</span> play_buffer
<span class="hljs-keyword">with</span> wave.open(<span class="hljs-string">'test.wav'</span>) <span class="hljs-keyword">as</span> file:
    frames, p = file.readframes(<span class="hljs-number">-1</span>), file.getparams()
    play_buffer(frames, p.nchannels, p.sampwidth, p.framerate).wait_done()
</code></pre></div>

<div><h3 id="texttospeech">Text to Speech</h3><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install pyttsx3</span>
<span class="hljs-keyword">import</span> pyttsx3
engine = pyttsx3.init()
engine.say(<span class="hljs-string">'Sally sells seashells by the seashore.'</span>)
engine.runAndWait()
</code></pre></div>

<div><h2 id="synthesizer"><a href="#synthesizer" name="synthesizer">#</a>Synthesizer</h2><div><h4 id="playspopcornbygershonkingsley">Plays Popcorn by Gershon Kingsley:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install simpleaudio</span>
<span class="hljs-keyword">import</span> itertools <span class="hljs-keyword">as</span> it, math, array, simpleaudio

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">play_notes</span><span class="hljs-params">(notes, bpm=<span class="hljs-number">132</span>, f=<span class="hljs-number">44100</span>)</span>:</span>
    get_pause   = <span class="hljs-keyword">lambda</span> n_beats: it.repeat(<span class="hljs-number">0</span>, int(n_beats * <span class="hljs-number">60</span>/bpm * f))
    sin_f       = <span class="hljs-keyword">lambda</span> i, hz: math.sin(i * <span class="hljs-number">2</span> * math.pi * hz / f)
    get_wave    = <span class="hljs-keyword">lambda</span> hz, n_beats: (sin_f(i, hz) <span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> range(int(n_beats * <span class="hljs-number">60</span>/bpm * f)))
    get_hz      = <span class="hljs-keyword">lambda</span> note: <span class="hljs-number">440</span> * <span class="hljs-number">2</span> ** ((int(note[:<span class="hljs-number">2</span>]) - <span class="hljs-number">69</span>) / <span class="hljs-number">12</span>)
    get_nbeats  = <span class="hljs-keyword">lambda</span> note: <span class="hljs-number">1</span>/<span class="hljs-number">2</span> <span class="hljs-keyword">if</span> <span class="hljs-string">'♩'</span> <span class="hljs-keyword">in</span> note <span class="hljs-keyword">else</span> <span class="hljs-number">1</span>/<span class="hljs-number">4</span> <span class="hljs-keyword">if</span> <span class="hljs-string">'♪'</span> <span class="hljs-keyword">in</span> note <span class="hljs-keyword">else</span> <span class="hljs-number">1</span>
    get_samples = <span class="hljs-keyword">lambda</span> n: get_wave(get_hz(n), get_nbeats(n)) <span class="hljs-keyword">if</span> n <span class="hljs-keyword">else</span> get_pause(<span class="hljs-number">1</span>/<span class="hljs-number">4</span>)
    samples_f   = it.chain.from_iterable(get_samples(n) <span class="hljs-keyword">for</span> n <span class="hljs-keyword">in</span> notes.split(<span class="hljs-string">','</span>))
    samples_i   = array.array(<span class="hljs-string">'h'</span>, (int(fl * <span class="hljs-number">5000</span>) <span class="hljs-keyword">for</span> fl <span class="hljs-keyword">in</span> samples_f))
    simpleaudio.play_buffer(samples_i, <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, f).wait_done()

play_notes(<span class="hljs-string">'83♩,81♪,,83♪,,78♪,,74♪,,78♪,,71♪,,,,83♪,,81♪,,83♪,,78♪,,74♪,,78♪,,71♪,,,,'</span>
           <span class="hljs-string">'83♩,85♪,,86♪,,85♪,,86♪,,83♪,,85♩,83♪,,85♪,,81♪,,83♪,,81♪,,83♪,,79♪,,83♪,,,,'</span>)
</code></pre></div></div>


<div><h2 id="pygame"><a href="#pygame" name="pygame">#</a>Pygame</h2><div><h4 id="opensawindowanddrawsasquarethatcanbemovedwitharrowkeys">Opens a window and draws a square that can be moved with arrow keys:</h4><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install pygame</span>
<span class="hljs-keyword">import</span> pygame <span class="hljs-keyword">as</span> pg

pg.init()
screen = pg.display.set_mode((<span class="hljs-number">500</span>, <span class="hljs-number">500</span>))
rect = pg.Rect(<span class="hljs-number">240</span>, <span class="hljs-number">240</span>, <span class="hljs-number">20</span>, <span class="hljs-number">20</span>)
<span class="hljs-keyword">while</span> <span class="hljs-keyword">not</span> pg.event.get(pg.QUIT):
    <span class="hljs-keyword">for</span> event <span class="hljs-keyword">in</span> pg.event.get(pg.KEYDOWN):
        dx = (event.key == pg.K_RIGHT) - (event.key == pg.K_LEFT)
        dy = (event.key == pg.K_DOWN) - (event.key == pg.K_UP)
        rect = rect.move((dx * <span class="hljs-number">20</span>, dy * <span class="hljs-number">20</span>))
    screen.fill(pg.Color(<span class="hljs-string">'black'</span>))
    pg.draw.rect(screen, pg.Color(<span class="hljs-string">'white'</span>), rect)
    pg.display.flip()
pg.quit()
</code></pre></div></div>


<div><h3 id="rect">Rect</h3><p><strong>Object for storing rectangular coordinates.</strong></p><pre><code class="python language-python hljs">&lt;Rect&gt; = pg.Rect(x, y, width, height)           <span class="hljs-comment"># Creates Rect object. Truncates passed floats.</span>
&lt;int&gt;  = &lt;Rect&gt;.x/y/centerx/centery/…           <span class="hljs-comment"># `top/right/bottom/left`. Allows assignments.</span>
&lt;tup.&gt; = &lt;Rect&gt;.topleft/center/…                <span class="hljs-comment"># `topright/bottomright/bottomleft/size`. Same.</span>
&lt;Rect&gt; = &lt;Rect&gt;.move((delta_x, delta_y))        <span class="hljs-comment"># Use move_ip() to move the rectangle in-place.</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;bool&gt; = &lt;Rect&gt;.collidepoint((x, y))            <span class="hljs-comment"># Checks whether rectangle contains the point.</span>
&lt;bool&gt; = &lt;Rect&gt;.colliderect(&lt;Rect&gt;)             <span class="hljs-comment"># Checks whether the two rectangles overlap.</span>
&lt;int&gt;  = &lt;Rect&gt;.collidelist(&lt;list_of_Rect&gt;)     <span class="hljs-comment"># Returns index of first colliding Rect or -1.</span>
&lt;list&gt; = &lt;Rect&gt;.collidelistall(&lt;list_of_Rect&gt;)  <span class="hljs-comment"># Returns indices of all colliding rectangles.</span>
</code></pre>
<div><h3 id="surface">Surface</h3><p><strong>Object for representing images.</strong></p><pre><code class="python language-python hljs">&lt;Surf&gt; = pg.display.set_mode((width, height))   <span class="hljs-comment"># Opens a new window and returns its surface.</span>
&lt;Surf&gt; = pg.Surface((width, height))            <span class="hljs-comment"># New RGB surface. RGBA if `flags=pg.SRCALPHA`.</span>
&lt;Surf&gt; = pg.image.load(&lt;path/file&gt;)             <span class="hljs-comment"># Loads the image. Format depends on source.</span>
&lt;Surf&gt; = pg.surfarray.make_surface(&lt;np_array&gt;)  <span class="hljs-comment"># Also `&lt;np_arr&gt; = surfarray.pixels3d(&lt;Surf&gt;)`.</span>
&lt;Surf&gt; = &lt;Surf&gt;.subsurface(&lt;Rect&gt;)              <span class="hljs-comment"># Creates a new surface from the cutout.</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;Surf&gt;.fill(color)                              <span class="hljs-comment"># Pass tuple of ints or pg.Color('&lt;name/hex&gt;').</span>
&lt;Surf&gt;.set_at((x, y), color)                    <span class="hljs-comment"># Updates pixel. Also &lt;Surf&gt;.get_at((x, y)).</span>
&lt;Surf&gt;.blit(&lt;Surf&gt;, (x, y))                     <span class="hljs-comment"># Draws passed surface at specified location.</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> pygame.transform <span class="hljs-keyword">import</span> scale, rotate      <span class="hljs-comment"># Also: flip, smoothscale, scale_by.</span>
&lt;Surf&gt; = scale(&lt;Surf&gt;, (width, height))         <span class="hljs-comment"># Scales the surface. `smoothscale()` blurs it.</span>
&lt;Surf&gt; = rotate(&lt;Surf&gt;, angle)                  <span class="hljs-comment"># Rotates the surface for counterclock degrees.</span>
&lt;Surf&gt; = flip(&lt;Surf&gt;, flip_x=<span class="hljs-keyword">False</span>)             <span class="hljs-comment"># Mirrors the surface. Also `flip_y=False`.</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-keyword">from</span> pygame.draw <span class="hljs-keyword">import</span> line, arc, rect         <span class="hljs-comment"># Also: ellipse, polygon, circle, aaline.</span>
line(&lt;Surf&gt;, color, (x1, y1), (x2, y2))         <span class="hljs-comment"># Draws a line to the surface. Also `width=1`.</span>
arc(&lt;Surf&gt;, color, &lt;Rect&gt;, from_rad, to_rad)    <span class="hljs-comment"># Also ellipse(&lt;Surf&gt;, color, &lt;Rect&gt;, width=0).</span>
rect(&lt;Surf&gt;, color, &lt;Rect&gt;, width=<span class="hljs-number">0</span>)            <span class="hljs-comment"># Also polygon(&lt;Surf&gt;, color, points, width=0).</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Font&gt; = pg.font.Font(&lt;path/file&gt;, size)        <span class="hljs-comment"># Loads TTF file. Pass None for default font.</span>
&lt;Surf&gt; = &lt;Font&gt;.render(text, antialias, color)  <span class="hljs-comment"># Accepts background color as fourth argument.</span>
</code></pre>
<div><h3 id="sound">Sound</h3><pre><code class="python language-python hljs">&lt;Sound&gt; = pg.mixer.Sound(&lt;path/file/bytes&gt;)     <span class="hljs-comment"># WAV file or bytes/array of signed shorts.</span>
&lt;Sound&gt;.play/stop()                             <span class="hljs-comment"># Also set_volume(&lt;float&gt;) and fadeout(msec).</span>
</code></pre></div>

<div><h3 id="basicmariobrothersexample">Basic Mario Brothers Example</h3><pre><code class="python language-python hljs"><span class="hljs-keyword">import</span> collections, dataclasses, enum, io, itertools <span class="hljs-keyword">as</span> it, pygame <span class="hljs-keyword">as</span> pg, urllib.request
<span class="hljs-keyword">from</span> random <span class="hljs-keyword">import</span> randint

P = collections.namedtuple(<span class="hljs-string">'P'</span>, <span class="hljs-string">'x y'</span>)          <span class="hljs-comment"># Position</span>
D = enum.Enum(<span class="hljs-string">'D'</span>, <span class="hljs-string">'n e s w'</span>)                   <span class="hljs-comment"># Direction</span>
W, H, MAX_S = <span class="hljs-number">50</span>, <span class="hljs-number">50</span>, P(<span class="hljs-number">5</span>, <span class="hljs-number">10</span>)                  <span class="hljs-comment"># Width, Height, Max speed</span>

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main</span><span class="hljs-params">()</span>:</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_screen</span><span class="hljs-params">()</span>:</span>
        pg.init()
        <span class="hljs-keyword">return</span> pg.display.set_mode((W*<span class="hljs-number">16</span>, H*<span class="hljs-number">16</span>))
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_images</span><span class="hljs-params">()</span>:</span>
        url = <span class="hljs-string">'https://gto76.github.io/python-cheatsheet/web/mario_bros.png'</span>
        img = pg.image.load(io.BytesIO(urllib.request.urlopen(url).read()))
        <span class="hljs-keyword">return</span> [img.subsurface(get_rect(x, <span class="hljs-number">0</span>)) <span class="hljs-keyword">for</span> x <span class="hljs-keyword">in</span> range(img.get_width() // <span class="hljs-number">16</span>)]
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_mario</span><span class="hljs-params">()</span>:</span>
        Mario = dataclasses.make_dataclass(<span class="hljs-string">'Mario'</span>, <span class="hljs-string">'rect spd facing_left frame_cycle'</span>.split())
        <span class="hljs-keyword">return</span> Mario(get_rect(<span class="hljs-number">1</span>, <span class="hljs-number">1</span>), P(<span class="hljs-number">0</span>, <span class="hljs-number">0</span>), <span class="hljs-keyword">False</span>, it.cycle(range(<span class="hljs-number">3</span>)))
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_tiles</span><span class="hljs-params">()</span>:</span>
        border = [(x, y) <span class="hljs-keyword">for</span> x <span class="hljs-keyword">in</span> range(W) <span class="hljs-keyword">for</span> y <span class="hljs-keyword">in</span> range(H) <span class="hljs-keyword">if</span> x <span class="hljs-keyword">in</span> [<span class="hljs-number">0</span>, W-<span class="hljs-number">1</span>] <span class="hljs-keyword">or</span> y <span class="hljs-keyword">in</span> [<span class="hljs-number">0</span>, H-<span class="hljs-number">1</span>]]
        platforms = [(randint(<span class="hljs-number">1</span>, W-<span class="hljs-number">2</span>), randint(<span class="hljs-number">2</span>, H-<span class="hljs-number">2</span>)) <span class="hljs-keyword">for</span> _ <span class="hljs-keyword">in</span> range(W*H // <span class="hljs-number">10</span>)]
        <span class="hljs-keyword">return</span> [get_rect(x, y) <span class="hljs-keyword">for</span> x, y <span class="hljs-keyword">in</span> border + platforms]
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_rect</span><span class="hljs-params">(x, y)</span>:</span>
        <span class="hljs-keyword">return</span> pg.Rect(x*<span class="hljs-number">16</span>, y*<span class="hljs-number">16</span>, <span class="hljs-number">16</span>, <span class="hljs-number">16</span>)
    run(get_screen(), get_images(), get_mario(), get_tiles())

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">run</span><span class="hljs-params">(screen, images, mario, tiles)</span>:</span>
    clock = pg.time.Clock()
    pressed = set()
    <span class="hljs-keyword">while</span> <span class="hljs-keyword">not</span> pg.event.get(pg.QUIT):
        clock.tick(<span class="hljs-number">28</span>)
        pressed |= {e.key <span class="hljs-keyword">for</span> e <span class="hljs-keyword">in</span> pg.event.get(pg.KEYDOWN)}
        pressed -= {e.key <span class="hljs-keyword">for</span> e <span class="hljs-keyword">in</span> pg.event.get(pg.KEYUP)}
        update_speed(mario, tiles, pressed)
        update_position(mario, tiles)
        draw(screen, images, mario, tiles)

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">update_speed</span><span class="hljs-params">(mario, tiles, pressed)</span>:</span>
    x, y = mario.spd
    x += <span class="hljs-number">2</span> * ((pg.K_RIGHT <span class="hljs-keyword">in</span> pressed) - (pg.K_LEFT <span class="hljs-keyword">in</span> pressed))
    x += (x &lt; <span class="hljs-number">0</span>) - (x &gt; <span class="hljs-number">0</span>)
    y += <span class="hljs-number">1</span> <span class="hljs-keyword">if</span> D.s <span class="hljs-keyword">not</span> <span class="hljs-keyword">in</span> get_boundaries(mario.rect, tiles) <span class="hljs-keyword">else</span> (pg.K_UP <span class="hljs-keyword">in</span> pressed) * <span class="hljs-number">-10</span>
    mario.spd = P(x=max(-MAX_S.x, min(MAX_S.x, x)), y=max(-MAX_S.y, min(MAX_S.y, y)))

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">update_position</span><span class="hljs-params">(mario, tiles)</span>:</span>
    x, y = mario.rect.topleft
    n_steps = max(abs(s) <span class="hljs-keyword">for</span> s <span class="hljs-keyword">in</span> mario.spd)
    <span class="hljs-keyword">for</span> _ <span class="hljs-keyword">in</span> range(n_steps):
        mario.spd = stop_on_collision(mario.spd, get_boundaries(mario.rect, tiles))
        x, y = x + (mario.spd.x / n_steps), y + (mario.spd.y / n_steps)
        mario.rect.topleft = x, y

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_boundaries</span><span class="hljs-params">(rect, tiles)</span>:</span>
    deltas = {D.n: P(<span class="hljs-number">0</span>, <span class="hljs-number">-1</span>), D.e: P(<span class="hljs-number">1</span>, <span class="hljs-number">0</span>), D.s: P(<span class="hljs-number">0</span>, <span class="hljs-number">1</span>), D.w: P(<span class="hljs-number">-1</span>, <span class="hljs-number">0</span>)}
    <span class="hljs-keyword">return</span> {d <span class="hljs-keyword">for</span> d, delta <span class="hljs-keyword">in</span> deltas.items() <span class="hljs-keyword">if</span> rect.move(delta).collidelist(tiles) != <span class="hljs-number">-1</span>}

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">stop_on_collision</span><span class="hljs-params">(spd, bounds)</span>:</span>
    <span class="hljs-keyword">return</span> P(x=<span class="hljs-number">0</span> <span class="hljs-keyword">if</span> (D.w <span class="hljs-keyword">in</span> bounds <span class="hljs-keyword">and</span> spd.x &lt; <span class="hljs-number">0</span>) <span class="hljs-keyword">or</span> (D.e <span class="hljs-keyword">in</span> bounds <span class="hljs-keyword">and</span> spd.x &gt; <span class="hljs-number">0</span>) <span class="hljs-keyword">else</span> spd.x,
             y=<span class="hljs-number">0</span> <span class="hljs-keyword">if</span> (D.n <span class="hljs-keyword">in</span> bounds <span class="hljs-keyword">and</span> spd.y &lt; <span class="hljs-number">0</span>) <span class="hljs-keyword">or</span> (D.s <span class="hljs-keyword">in</span> bounds <span class="hljs-keyword">and</span> spd.y &gt; <span class="hljs-number">0</span>) <span class="hljs-keyword">else</span> spd.y)

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">draw</span><span class="hljs-params">(screen, images, mario, tiles)</span>:</span>
    screen.fill((<span class="hljs-number">85</span>, <span class="hljs-number">168</span>, <span class="hljs-number">255</span>))
    mario.facing_left = mario.spd.x &lt; <span class="hljs-number">0</span> <span class="hljs-keyword">if</span> mario.spd.x <span class="hljs-keyword">else</span> mario.facing_left
    is_airborne = D.s <span class="hljs-keyword">not</span> <span class="hljs-keyword">in</span> get_boundaries(mario.rect, tiles)
    image_index = <span class="hljs-number">4</span> <span class="hljs-keyword">if</span> is_airborne <span class="hljs-keyword">else</span> next(mario.frame_cycle) <span class="hljs-keyword">if</span> mario.spd.x <span class="hljs-keyword">else</span> <span class="hljs-number">6</span>
    screen.blit(images[image_index + (mario.facing_left * <span class="hljs-number">9</span>)], mario.rect)
    <span class="hljs-keyword">for</span> t <span class="hljs-keyword">in</span> tiles:
        is_border = t.x <span class="hljs-keyword">in</span> [<span class="hljs-number">0</span>, (W-<span class="hljs-number">1</span>)*<span class="hljs-number">16</span>] <span class="hljs-keyword">or</span> t.y <span class="hljs-keyword">in</span> [<span class="hljs-number">0</span>, (H-<span class="hljs-number">1</span>)*<span class="hljs-number">16</span>]
        screen.blit(images[<span class="hljs-number">18</span> <span class="hljs-keyword">if</span> is_border <span class="hljs-keyword">else</span> <span class="hljs-number">19</span>], t)
    pg.display.flip()

<span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:
    main()
</code></pre></div>

<div><h2 id="pandas"><a href="#pandas" name="pandas">#</a>Pandas</h2><p><strong>Data analysis library. For examples see <a href="#plotly">Plotly</a>.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install pandas matplotlib</span>
<span class="hljs-keyword">import</span> pandas <span class="hljs-keyword">as</span> pd, matplotlib.pyplot <span class="hljs-keyword">as</span> plt
</code></pre></div>


<div><h3 id="series">Series</h3><p><strong>Ordered dictionary with a name.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>s = pd.Series([<span class="hljs-number">1</span>, <span class="hljs-number">2</span>], index=[<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>], name=<span class="hljs-string">'a'</span>); s
x    <span class="hljs-number">1</span>
y    <span class="hljs-number">2</span>
Name: a, dtype: int64
</code></pre></div>


<pre><code class="python language-python hljs">&lt;S&gt;  = pd.Series(&lt;list&gt;)                       <span class="hljs-comment"># Uses list's indices for 'index'.</span>
&lt;S&gt;  = pd.Series(&lt;dict&gt;)                       <span class="hljs-comment"># Uses dictionary's keys for 'index'.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;el&gt; = &lt;S&gt;.loc[key]                            <span class="hljs-comment"># Or: &lt;S&gt;.iloc[i]</span>
&lt;S&gt;  = &lt;S&gt;.loc[coll_of_keys]                   <span class="hljs-comment"># Or: &lt;S&gt;.iloc[coll_of_i]</span>
&lt;S&gt;  = &lt;S&gt;.loc[from_key : to_key_inc]          <span class="hljs-comment"># Or: &lt;S&gt;.iloc[from_i : to_i_exc]</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;el&gt; = &lt;S&gt;[key/i]                              <span class="hljs-comment"># Or: &lt;S&gt;.&lt;key&gt;</span>
&lt;S&gt;  = &lt;S&gt;[coll_of_keys/coll_of_i]             <span class="hljs-comment"># Or: &lt;S&gt;[key/i : key/i]</span>
&lt;S&gt;  = &lt;S&gt;[&lt;S_of_bools&gt;]                       <span class="hljs-comment"># Or: &lt;S&gt;.loc/iloc[&lt;S_of_bools&gt;]</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;S&gt;  = &lt;S&gt; &gt; &lt;el/S&gt;                            <span class="hljs-comment"># Returns S of bools. For logic use &amp;, |, ~.</span>
&lt;S&gt;  = &lt;S&gt; + &lt;el/S&gt;                            <span class="hljs-comment"># Items with non-matching keys get value NaN.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;S&gt;  = &lt;S&gt;.head/describe/sort_values()         <span class="hljs-comment"># Also &lt;S&gt;.unique/value_counts/round/dropna().</span>
&lt;S&gt;  = &lt;S&gt;.str.strip/lower/contains/replace()  <span class="hljs-comment"># Also split().str[i] or split(expand=True).</span>
&lt;S&gt;  = &lt;S&gt;.dt.year/month/day/hour              <span class="hljs-comment"># Use pd.to_datetime(&lt;S&gt;) to get S of datetimes.</span>
&lt;S&gt;  = &lt;S&gt;.dt.to_period(<span class="hljs-string">'y/m/d/h'</span>)             <span class="hljs-comment"># Quantizes datetimes into Period objects.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;S&gt;.plot.line/area/bar/pie/hist()              <span class="hljs-comment"># Generates a plot. Accepts `title=&lt;str&gt;`.</span>
plt.show()                                     <span class="hljs-comment"># Displays the plot. Also plt.savefig(&lt;path&gt;).</span>
</code></pre>
<ul>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'print(&lt;S&gt;.to_string())'</span></code> to print a Series that has more than sixty items.</strong></li>
<li><strong>Use <code class="python hljs"><span class="hljs-string">'&lt;S&gt;.index'</span></code> to get collection of keys and <code class="python hljs"><span class="hljs-string">'&lt;S&gt;.index = &lt;coll&gt;'</span></code> to update them.</strong></li>
<li><strong>Only pass a list or Series to loc/iloc because <code class="python hljs"><span class="hljs-string">'obj[x, y]'</span></code> is converted to <code class="python hljs"><span class="hljs-string">'obj[(x, y)]'</span></code> and <code class="python hljs"><span class="hljs-string">'&lt;S&gt;.loc[key_1, key_2]'</span></code> is how you retrieve a value from a multi-indexed Series.</strong></li>
<li><strong>Pandas uses NumPy types like <code class="python hljs"><span class="hljs-string">'np.int64'</span></code>. Series is converted to <code class="python hljs"><span class="hljs-string">'float64'</span></code> if np.nan is assigned to any item. Use <code class="python hljs"><span class="hljs-string">'&lt;S&gt;.astype(&lt;str/type&gt;)'</span></code> to get converted Series.</strong></li>
</ul>
<div><h4 id="seriesaggregatetransformmap">Series — Aggregate, Transform, Map:</h4><pre><code class="python language-python hljs">&lt;el&gt; = &lt;S&gt;.sum/max/mean/std/idxmax/count()     <span class="hljs-comment"># Or: &lt;S&gt;.agg(lambda &lt;S&gt;: &lt;el&gt;)</span>
&lt;S&gt;  = &lt;S&gt;.rank/diff/cumsum/ffill/interpol…()  <span class="hljs-comment"># Or: &lt;S&gt;.agg/transform(lambda &lt;S&gt;: &lt;S&gt;)</span>
&lt;S&gt;  = &lt;S&gt;.isna/fillna/isin([&lt;el/coll&gt;])       <span class="hljs-comment"># Or: &lt;S&gt;.agg/transform/map(lambda &lt;el&gt;: &lt;el&gt;)</span>
</code></pre></div>

<pre><code class="python hljs">┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━┯━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┓
┃              │    <span class="hljs-string">'sum'</span>    │   [<span class="hljs-string">'sum'</span>]   │ {<span class="hljs-string">'s'</span>: <span class="hljs-string">'sum'</span>}  ┃
┠──────────────┼─────────────┼─────────────┼───────────────┨
┃ s.apply(…)   │      <span class="hljs-number">3</span>      │    sum  <span class="hljs-number">3</span>   │     s  <span class="hljs-number">3</span>      ┃
┃ s.agg(…)     │             │             │               ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━┷━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┛

┏━━━━━━━━━━━━━━┯━━━━━━━━━━━━━┯━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┓
┃              │    <span class="hljs-string">'rank'</span>   │   [<span class="hljs-string">'rank'</span>]  │ {<span class="hljs-string">'r'</span>: <span class="hljs-string">'rank'</span>} ┃
┠──────────────┼─────────────┼─────────────┼───────────────┨
┃ s.apply(…)   │             │      rank   │               ┃
┃ s.agg(…)     │    x  <span class="hljs-number">1.0</span>   │   x   <span class="hljs-number">1.0</span>   │   r  x  <span class="hljs-number">1.0</span>   ┃
┃              │    y  <span class="hljs-number">2.0</span>   │   y   <span class="hljs-number">2.0</span>   │      y  <span class="hljs-number">2.0</span>   ┃
┗━━━━━━━━━━━━━━┷━━━━━━━━━━━━━┷━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┛
</code></pre>

<div><h3 id="dataframe">DataFrame</h3><p><strong>Table with labeled rows and columns.</strong></p><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>df = pd.DataFrame([[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>], [<span class="hljs-number">3</span>, <span class="hljs-number">4</span>]], index=[<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>], columns=[<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>]); df
   x  y
a  <span class="hljs-number">1</span>  <span class="hljs-number">2</span>
b  <span class="hljs-number">3</span>  <span class="hljs-number">4</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;DF&gt;   = pd.DataFrame(&lt;list_of_rows&gt;)          <span class="hljs-comment"># Rows can be either lists, dicts or series.</span>
&lt;DF&gt;   = pd.DataFrame(&lt;dict_of_columns&gt;)       <span class="hljs-comment"># Columns can be either lists, dicts or series.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;el&gt;   = &lt;DF&gt;.loc[row_key, col_key]            <span class="hljs-comment"># Or: &lt;DF&gt;.iloc[row_i, col_i]</span>
&lt;S/DF&gt; = &lt;DF&gt;.loc[row_key/s]                   <span class="hljs-comment"># Or: &lt;DF&gt;.iloc[row_i/s]</span>
&lt;S/DF&gt; = &lt;DF&gt;.loc[:, col_key/s]                <span class="hljs-comment"># Or: &lt;DF&gt;.iloc[:, col_i/s]</span>
&lt;DF&gt;   = &lt;DF&gt;.loc[row_bools, col_bools]        <span class="hljs-comment"># Or: &lt;DF&gt;.iloc[row_bools, col_bools]</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;S/DF&gt; = &lt;DF&gt;[col_key/s]                       <span class="hljs-comment"># Or: &lt;DF&gt;.&lt;col_key&gt;</span>
&lt;DF&gt;   = &lt;DF&gt;[&lt;S_of_bools&gt;]                    <span class="hljs-comment"># Filters rows. For example `df[df.x &gt; 1]`.</span>
&lt;DF&gt;   = &lt;DF&gt;[&lt;DF_of_bools&gt;]                   <span class="hljs-comment"># Assigns NaN to items that are False in bools.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;DF&gt;   = &lt;DF&gt; &gt; &lt;el/S/DF&gt;                      <span class="hljs-comment"># Returns DF of bools. Treats series as a row.</span>
&lt;DF&gt;   = &lt;DF&gt; + &lt;el/S/DF&gt;                      <span class="hljs-comment"># Items with non-matching keys get value NaN.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;DF&gt;   = &lt;DF&gt;.set_index(col_key)               <span class="hljs-comment"># Replaces row keys with column's values.</span>
&lt;DF&gt;   = &lt;DF&gt;.reset_index(drop=<span class="hljs-keyword">False</span>)          <span class="hljs-comment"># Drops or moves row keys to column named index.</span>
&lt;DF&gt;   = &lt;DF&gt;.sort_index(ascending=<span class="hljs-keyword">True</span>)       <span class="hljs-comment"># Sorts rows by row keys. Use `axis=1` for cols.</span>
&lt;DF&gt;   = &lt;DF&gt;.sort_values(col_key/s)           <span class="hljs-comment"># Sorts rows by passed column/s. Also `axis=1`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;DF&gt;   = &lt;DF&gt;.head/tail/sample(&lt;int&gt;)          <span class="hljs-comment"># Returns first, last, or random n rows.</span>
&lt;DF&gt;   = &lt;DF&gt;.describe()                       <span class="hljs-comment"># Describes columns. Also info(), corr(), shape.</span>
&lt;DF&gt;   = &lt;DF&gt;.query(<span class="hljs-string">'&lt;query&gt;'</span>)                 <span class="hljs-comment"># Filters rows. For example `df.query('x &gt; 1')`.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;DF&gt;.plot.line/area/bar/scatter(x=col_key, …)  <span class="hljs-comment"># `y=col_key/s`. Also hist/box(column/by=col_k).</span>
plt.show()                                     <span class="hljs-comment"># Displays the plot. Also plt.savefig(&lt;path&gt;).</span>
</code></pre>
<div><h4 id="dataframemergejoinconcat">DataFrame — Merge, Join, Concat:</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>df_2 = pd.DataFrame([[<span class="hljs-number">4</span>, <span class="hljs-number">5</span>], [<span class="hljs-number">6</span>, <span class="hljs-number">7</span>]], index=[<span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>], columns=[<span class="hljs-string">'y'</span>, <span class="hljs-string">'z'</span>]); df_2
   y  z
b  <span class="hljs-number">4</span>  <span class="hljs-number">5</span>
c  <span class="hljs-number">6</span>  <span class="hljs-number">7</span>
</code></pre></div>

<pre><code class="python hljs">┏━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃                       │    <span class="hljs-string">'outer'</span>    │   <span class="hljs-string">'inner'</span>  │   <span class="hljs-string">'left'</span>   │       Description         ┃
┠───────────────────────┼───────────────┼────────────┼────────────┼───────────────────────────┨
┃ df.merge(df_2,        │    x   y   z  │ x   y   z  │ x   y   z  │ Merges on column if 'on'  ┃
┃          on=<span class="hljs-string">'y'</span>,      │ <span class="hljs-number">0</span>  <span class="hljs-number">1</span>   <span class="hljs-number">2</span>   .  │ <span class="hljs-number">3</span>   <span class="hljs-number">4</span>   <span class="hljs-number">5</span>  │ <span class="hljs-number">1</span>   <span class="hljs-number">2</span>   .  │ or 'left_on/right_on' are ┃
┃          how=…)       │ <span class="hljs-number">1</span>  <span class="hljs-number">3</span>   <span class="hljs-number">4</span>   <span class="hljs-number">5</span>  │            │ <span class="hljs-number">3</span>   <span class="hljs-number">4</span>   <span class="hljs-number">5</span>  │ set, else on shared cols. ┃
┃                       │ <span class="hljs-number">2</span>  .   <span class="hljs-number">6</span>   <span class="hljs-number">7</span>  │            │            │ Uses <span class="hljs-string">'inner'</span> by default.  ┃
┠───────────────────────┼───────────────┼────────────┼────────────┼───────────────────────────┨
┃ df.join(df_2,         │    x yl yr  z │            │ x yl yr  z │ Merges on row keys.       ┃
┃         lsuffix=<span class="hljs-string">'l'</span>,  │ a  <span class="hljs-number">1</span>  <span class="hljs-number">2</span>  .  . │ x yl yr  z │ <span class="hljs-number">1</span>  <span class="hljs-number">2</span>  .  . │ Uses <span class="hljs-string">'left'</span> by default.   ┃
┃         rsuffix=<span class="hljs-string">'r'</span>,  │ b  <span class="hljs-number">3</span>  <span class="hljs-number">4</span>  <span class="hljs-number">4</span>  <span class="hljs-number">5</span> │ <span class="hljs-number">3</span>  <span class="hljs-number">4</span>  <span class="hljs-number">4</span>  <span class="hljs-number">5</span> │ <span class="hljs-number">3</span>  <span class="hljs-number">4</span>  <span class="hljs-number">4</span>  <span class="hljs-number">5</span> │ If Series is passed, it   ┃
┃         how=…)        │ c  .  .  <span class="hljs-number">6</span>  <span class="hljs-number">7</span> │            │            │ is treated as a column.   ┃
┠───────────────────────┼───────────────┼────────────┼────────────┼───────────────────────────┨
┃ pd.concat([df, df_2], │    x   y   z  │     y      │            │ Adds rows at the bottom.  ┃
┃           axis=<span class="hljs-number">0</span>,     │ a  <span class="hljs-number">1</span>   <span class="hljs-number">2</span>   .  │     <span class="hljs-number">2</span>      │            │ Uses <span class="hljs-string">'outer'</span> by default.  ┃
┃           join=…)     │ b  <span class="hljs-number">3</span>   <span class="hljs-number">4</span>   .  │     <span class="hljs-number">4</span>      │            │ A Series is treated as a  ┃
┃                       │ b  .   <span class="hljs-number">4</span>   <span class="hljs-number">5</span>  │     <span class="hljs-number">4</span>      │            │ column. To add a row use  ┃
┃                       │ c  .   <span class="hljs-number">6</span>   <span class="hljs-number">7</span>  │     <span class="hljs-number">6</span>      │            │ pd.concat([df, DF([s])]). ┃
┠───────────────────────┼───────────────┼────────────┼────────────┼───────────────────────────┨
┃ pd.concat([df, df_2], │    x  y  y  z │            │            │ Adds columns at the       ┃
┃           axis=<span class="hljs-number">1</span>,     │ a  <span class="hljs-number">1</span>  <span class="hljs-number">2</span>  .  . │ x  y  y  z │            │ right end. Uses <span class="hljs-string">'outer'</span>   ┃
┃           join=…)     │ b  <span class="hljs-number">3</span>  <span class="hljs-number">4</span>  <span class="hljs-number">4</span>  <span class="hljs-number">5</span> │ <span class="hljs-number">3</span>  <span class="hljs-number">4</span>  <span class="hljs-number">4</span>  <span class="hljs-number">5</span> │            │ by default. A Series is   ┃
┃                       │ c  .  .  <span class="hljs-number">6</span>  <span class="hljs-number">7</span> │            │            │ treated as a column.      ┃
┗━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
</code></pre>
<div><h4 id="dataframeaggregatetransformmap">DataFrame — Aggregate, Transform, Map:</h4><pre><code class="python language-python hljs">&lt;S&gt;  = &lt;DF&gt;.sum/max/mean/std/idxmax/count()    <span class="hljs-comment"># Or: &lt;DF&gt;.apply/agg(lambda &lt;S&gt;: &lt;el&gt;)</span>
&lt;DF&gt; = &lt;DF&gt;.rank/diff/cumsum/ffill/interpo…()  <span class="hljs-comment"># Or: &lt;DF&gt;.apply/agg/transform(lambda &lt;S&gt;: &lt;S&gt;)</span>
&lt;DF&gt; = &lt;DF&gt;.isna/fillna/isin([&lt;el/coll&gt;])      <span class="hljs-comment"># Or: &lt;DF&gt;.applymap(lambda &lt;el&gt;: &lt;el&gt;)</span>
</code></pre></div>

<pre><code class="python hljs">┏━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┓
┃                 │     <span class="hljs-string">'sum'</span>     │    [<span class="hljs-string">'sum'</span>]    │ {<span class="hljs-string">'x'</span>: <span class="hljs-string">'sum'</span>}  ┃
┠─────────────────┼───────────────┼───────────────┼───────────────┨
┃ df.apply(…)     │      x  <span class="hljs-number">4</span>     │        x  y   │     x  <span class="hljs-number">4</span>      ┃
┃ df.agg(…)       │      y  <span class="hljs-number">6</span>     │   sum  <span class="hljs-number">4</span>  <span class="hljs-number">6</span>   │               ┃
┗━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┛

┏━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━┓
┃                 │     <span class="hljs-string">'rank'</span>    │    [<span class="hljs-string">'rank'</span>]   │ {<span class="hljs-string">'x'</span>: <span class="hljs-string">'rank'</span>} ┃
┠─────────────────┼───────────────┼───────────────┼───────────────┨
┃ df.apply(…)     │               │       x    y  │               ┃
┃ df.agg(…)       │       x    y  │    rank rank  │         x     ┃
┃ df.transform(…) │  a  <span class="hljs-number">1.0</span>  <span class="hljs-number">1.0</span>  │  a  <span class="hljs-number">1.0</span>  <span class="hljs-number">1.0</span>  │    a  <span class="hljs-number">1.0</span>     ┃
┃                 │  b  <span class="hljs-number">2.0</span>  <span class="hljs-number">2.0</span>  │  b  <span class="hljs-number">2.0</span>  <span class="hljs-number">2.0</span>  │    b  <span class="hljs-number">2.0</span>     ┃
┗━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━┛
</code></pre>

<ul>
<li><strong>Listed methods process the columns unless they receive <code class="python hljs"><span class="hljs-string">'axis=1'</span></code>. Exceptions to this rule are <code class="python hljs"><span class="hljs-string">'&lt;DF&gt;.dropna()'</span></code>, <code class="python hljs"><span class="hljs-string">'&lt;DF&gt;.drop(row_key/s)'</span></code> and <code class="python hljs"><span class="hljs-string">'&lt;DF&gt;.rename(&lt;dict/func&gt;)'</span></code>.</strong></li>
<li><strong>Fifth result's columns are indexed with a multi-index. This means we need a tuple of column keys to specify a column: <code class="python hljs"><span class="hljs-string">'&lt;DF&gt;.loc[row_key, (col_key_1, col_key_2)]'</span></code>.</strong></li>
</ul>
<div><h3 id="multiindex">Multi-Index</h3><pre><code class="python language-python hljs">&lt;DF&gt; = &lt;DF&gt;.loc[row_key_1]                     <span class="hljs-comment"># Or: &lt;DF&gt;.xs(row_key_1)</span>
&lt;DF&gt; = &lt;DF&gt;.loc[:, (slice(<span class="hljs-keyword">None</span>), col_key_2)]   <span class="hljs-comment"># Or: &lt;DF&gt;.xs(col_key_2, axis=1, level=1)</span>
&lt;DF&gt; = &lt;DF&gt;.set_index(col_keys)                <span class="hljs-comment"># Creates index from cols. Also `append=False`.</span>
&lt;DF&gt; = &lt;DF&gt;.pivot_table(index=col_key/s)       <span class="hljs-comment"># `columns=key/s, values=key/s, aggfunc='mean'`.</span>
&lt;S&gt;  = &lt;DF&gt;.stack/unstack(level=<span class="hljs-number">-1</span>)            <span class="hljs-comment"># Combines col keys with row keys or vice versa.</span>
</code></pre></div>

<div><h3 id="fileformats">File Formats</h3><pre><code class="python language-python hljs">&lt;S/DF&gt; = pd.read_json/pickle(&lt;path/url/file&gt;)  <span class="hljs-comment"># Also io.StringIO(&lt;str&gt;), io.BytesIO(&lt;bytes&gt;).</span>
&lt;DF&gt;   = pd.read_csv/excel(&lt;path/url/file&gt;)    <span class="hljs-comment"># Also `header/index_col/dtype/usecols/…=&lt;obj&gt;`.</span>
&lt;list&gt; = pd.read_html(&lt;path/url/file&gt;)         <span class="hljs-comment"># Raises ImportError if webpage has zero tables.</span>
&lt;S/DF&gt; = pd.read_parquet/feather/hdf(&lt;path…&gt;)  <span class="hljs-comment"># Function read_hdf() accepts `key=&lt;s/df_name&gt;`.</span>
&lt;DF&gt;   = pd.read_sql(<span class="hljs-string">'&lt;table/query&gt;'</span>, &lt;conn&gt;)  <span class="hljs-comment"># Pass SQLite3/Alchemy connection. See #SQLite.</span>
</code></pre></div>

<pre><code class="python language-python hljs">&lt;DF&gt;.to_json/csv/html/latex/parquet(&lt;path&gt;)    <span class="hljs-comment"># Returns a string/bytes if path is omitted.</span>
&lt;DF&gt;.to_pickle/excel/feather/hdf(&lt;path&gt;)       <span class="hljs-comment"># Method to_hdf() requires `key=&lt;s/df_name&gt;`.</span>
&lt;DF&gt;.to_sql(<span class="hljs-string">'&lt;table_name&gt;'</span>, &lt;connection&gt;)      <span class="hljs-comment"># Also `if_exists='fail/replace/append'`.</span>
</code></pre>
<ul>
<li><strong><code class="python hljs"><span class="hljs-string">'$ pip3 install "pandas[excel]" odfpy lxml pyarrow'</span></code> installs dependencies.</strong></li>
<li><strong>Csv functions use the same dialect as standard library's csv module (e.g. <code class="python hljs"><span class="hljs-string">'sep=","'</span></code>).</strong></li>
<li><strong>Read_csv() only parses dates of columns that are listed in 'parse_dates'. It automatically tries to detect the format, but it can be helped with 'date_format' or 'dayfirst' arguments.</strong></li>
<li><strong>We get a dataframe with DatetimeIndex if 'parse_dates' argument includes 'index_col'. Its <code class="python hljs"><span class="hljs-string">'resample("y/m/d/h")'</span></code> method returns Resampler object that is similar to GroupBy.</strong></li>
</ul>
<div><h3 id="groupby">GroupBy</h3><p><strong>Object that groups together rows of a dataframe based on the value of the passed column.</strong></p><pre><code class="python language-python hljs">&lt;GB&gt; = &lt;DF&gt;.groupby(col_key/s)                 <span class="hljs-comment"># Splits DF into groups based on passed column.</span>
&lt;DF&gt; = &lt;GB&gt;.apply/filter(&lt;func&gt;)               <span class="hljs-comment"># Filter drops a group if func returns False.</span>
&lt;DF&gt; = &lt;GB&gt;.get_group(&lt;el&gt;)                    <span class="hljs-comment"># Selects a group by grouping column's value.</span>
&lt;S&gt;  = &lt;GB&gt;.size()                             <span class="hljs-comment"># S of group sizes. Same keys as get_group().</span>
&lt;GB&gt; = &lt;GB&gt;[col_key]                           <span class="hljs-comment"># Single column GB. All operations return S.</span>
</code></pre></div>


<pre><code class="python language-python hljs">&lt;DF&gt; = &lt;GB&gt;.sum/max/mean/std/idxmax/count()    <span class="hljs-comment"># Or: &lt;GB&gt;.agg(lambda &lt;S&gt;: &lt;el&gt;)</span>
&lt;DF&gt; = &lt;GB&gt;.rank/diff/cumsum/ffill()           <span class="hljs-comment"># Or: &lt;GB&gt;.transform(lambda &lt;S&gt;: &lt;S&gt;)</span>
&lt;DF&gt; = &lt;GB&gt;.fillna(&lt;el&gt;)                       <span class="hljs-comment"># Or: &lt;GB&gt;.transform(lambda &lt;S&gt;: &lt;S&gt;)</span>
</code></pre>
<div><h4 id="dividesrowsintogroupsandsumstheircolumnsresulthasanamedindexthatcreatescolumnzonreset_index">Divides rows into groups and sums their columns. Result has a named index that creates column <code class="python hljs"><span class="hljs-string">'z'</span></code> on reset_index():</h4><pre><code class="python language-python hljs"><span class="hljs-meta">&gt;&gt;&gt; </span>df = pd.DataFrame([[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>], [<span class="hljs-number">4</span>, <span class="hljs-number">5</span>, <span class="hljs-number">6</span>], [<span class="hljs-number">7</span>, <span class="hljs-number">8</span>, <span class="hljs-number">6</span>]], list(<span class="hljs-string">'abc'</span>), list(<span class="hljs-string">'xyz'</span>))
<span class="hljs-meta">&gt;&gt;&gt; </span>gb = df.groupby(<span class="hljs-string">'z'</span>); gb.apply(print)
   x  y  z
a  <span class="hljs-number">1</span>  <span class="hljs-number">2</span>  <span class="hljs-number">3</span>
   x  y  z
b  <span class="hljs-number">4</span>  <span class="hljs-number">5</span>  <span class="hljs-number">6</span>
c  <span class="hljs-number">7</span>  <span class="hljs-number">8</span>  <span class="hljs-number">6</span>
<span class="hljs-meta">&gt;&gt;&gt; </span>gb.sum()
    x   y
z
<span class="hljs-number">3</span>   <span class="hljs-number">1</span>   <span class="hljs-number">2</span>
<span class="hljs-number">6</span>  <span class="hljs-number">11</span>  <span class="hljs-number">13</span></code></pre></div>

<div><h3 id="rolling">Rolling</h3><p><strong>Object for rolling window calculations.</strong></p><pre><code class="python language-python hljs">&lt;RS/RDF/RGB&gt; = &lt;S/DF/GB&gt;.rolling(win_size)     <span class="hljs-comment"># Also: `min_periods=None, center=False`.</span>
&lt;RS/RDF/RGB&gt; = &lt;RDF/RGB&gt;[col_key/s]            <span class="hljs-comment"># Or: &lt;RDF/RGB&gt;.&lt;col_key&gt;</span>
&lt;S/DF&gt;       = &lt;R&gt;.mean/sum/max()              <span class="hljs-comment"># Or: &lt;R&gt;.apply/agg(&lt;agg_func/str&gt;)</span>
</code></pre></div>


<div><h2 id="plotly"><a href="#plotly" name="plotly">#</a>Plotly</h2><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install plotly kaleido pandas</span>
<span class="hljs-keyword">import</span> plotly.express <span class="hljs-keyword">as</span> px, pandas <span class="hljs-keyword">as</span> pd
</code></pre></div>

<pre><code class="python language-python hljs">&lt;Fig&gt; = px.line(&lt;DF&gt; [, y=col_key/s [, x=col_key]])   <span class="hljs-comment"># Also px.line(y=&lt;list&gt; [, x=&lt;list&gt;]).</span>
&lt;Fig&gt;.update_layout(paper_bgcolor=<span class="hljs-string">'#rrggbb'</span>)          <span class="hljs-comment"># Also `margin=dict(t=0, r=0, b=0, l=0)`.</span>
&lt;Fig&gt;.write_html/json/image(<span class="hljs-string">'&lt;path&gt;'</span>)                 <span class="hljs-comment"># Use &lt;Fig&gt;.show() to display the plot.</span>
</code></pre>
<pre><code class="python language-python hljs">&lt;Fig&gt; = px.area/bar/box(&lt;DF&gt;, x=col_key, y=col_keys)  <span class="hljs-comment"># Also `color=col_key`. All are optional.</span>
&lt;Fig&gt; = px.scatter(&lt;DF&gt;, x=col_key, y=col_keys)       <span class="hljs-comment"># Also `color/size/symbol=col_key`. Same.</span>
&lt;Fig&gt; = px.scatter_3d(&lt;DF&gt;, x=col_key, y=col_key, …)  <span class="hljs-comment"># `z=col_key`. Also color, size, symbol.</span>
&lt;Fig&gt; = px.histogram(&lt;DF&gt;, x=col_keys, y=col_key)     <span class="hljs-comment"># Also color, nbins. All are optional.</span>
</code></pre>
<div><h4 id="displaysalinechartoftotalcovid19deathspermilliongroupedbycontinent">Displays a line chart of total COVID-19 deaths per million grouped by continent:</h4><p></p><div id="2a950764-39fc-416d-97fe-0a6226a3095f" class="plotly-graph-div" style="height:312px; width:914px;"></div><pre><code class="python language-python hljs">covid = pd.read_csv(<span class="hljs-string">'https://raw.githubusercontent.com/owid/covid-19-data/8dde8ca49b'</span>
                    <span class="hljs-string">'6e648c17dd420b2726ca0779402651/public/data/owid-covid-data.csv'</span>,
                    usecols=[<span class="hljs-string">'iso_code'</span>, <span class="hljs-string">'date'</span>, <span class="hljs-string">'population'</span>, <span class="hljs-string">'total_deaths'</span>])
continents = pd.read_csv(<span class="hljs-string">'https://gto76.github.io/python-cheatsheet/web/continents.csv'</span>,
                         usecols=[<span class="hljs-string">'Three_Letter_Country_Code'</span>, <span class="hljs-string">'Continent_Name'</span>])
df = pd.merge(covid, continents, left_on=<span class="hljs-string">'iso_code'</span>, right_on=<span class="hljs-string">'Three_Letter_Country_Code'</span>)
df = df.groupby([<span class="hljs-string">'Continent_Name'</span>, <span class="hljs-string">'date'</span>]).sum().reset_index()
df[<span class="hljs-string">'Total Deaths per Million'</span>] = df.total_deaths * <span class="hljs-number">1e6</span> / df.population
df = df[df.date &gt; <span class="hljs-string">'2020-03-14'</span>]
df = df.rename({<span class="hljs-string">'date'</span>: <span class="hljs-string">'Date'</span>, <span class="hljs-string">'Continent_Name'</span>: <span class="hljs-string">'Continent'</span>}, axis=<span class="hljs-string">'columns'</span>)
px.line(df, x=<span class="hljs-string">'Date'</span>, y=<span class="hljs-string">'Total Deaths per Million'</span>, color=<span class="hljs-string">'Continent'</span>)
</code></pre></div>



<div><h4 id="displaysamultiaxislinechartoftotalcovid19casesandchangesinpricesofbitcoindowjonesandgold">Displays a multi-axis line chart of total COVID-19 cases and changes in prices of Bitcoin, Dow Jones and gold:</h4><p></p><div id="e23ccacc-a456-478b-b467-7282a2165921" class="plotly-graph-div" style="height:285px; width:935px;"></div><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install pandas lxml selenium plotly</span>
<span class="hljs-keyword">import</span> pandas <span class="hljs-keyword">as</span> pd, selenium.webdriver, io, plotly.graph_objects <span class="hljs-keyword">as</span> go

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main</span><span class="hljs-params">()</span>:</span>
    covid, (bitcoin, gold, dow) = get_covid_cases(), get_tickers()
    df = wrangle_data(covid, bitcoin, gold, dow)
    display_data(df)

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_covid_cases</span><span class="hljs-params">()</span>:</span>
    url = <span class="hljs-string">'https://catalog.ourworldindata.org/garden/covid/latest/compact/compact.csv'</span>
    df = pd.read_csv(url, parse_dates=[<span class="hljs-string">'date'</span>])
    df = df[df.country == <span class="hljs-string">'World'</span>]
    s = df.set_index(<span class="hljs-string">'date'</span>).total_cases
    <span class="hljs-keyword">return</span> s.rename(<span class="hljs-string">'Total Cases'</span>)

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_tickers</span><span class="hljs-params">()</span>:</span>
    <span class="hljs-keyword">with</span> selenium.webdriver.Chrome() <span class="hljs-keyword">as</span> driver:
        driver.implicitly_wait(<span class="hljs-number">10</span>)
        symbols = {<span class="hljs-string">'Bitcoin'</span>: <span class="hljs-string">'BTC-USD'</span>, <span class="hljs-string">'Gold'</span>: <span class="hljs-string">'GC=F'</span>, <span class="hljs-string">'Dow Jones'</span>: <span class="hljs-string">'%5EDJI'</span>}
        <span class="hljs-keyword">return</span> [get_ticker(driver, name, symbol) <span class="hljs-keyword">for</span> name, symbol <span class="hljs-keyword">in</span> symbols.items()]

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">get_ticker</span><span class="hljs-params">(driver, name, symbol)</span>:</span>
    url = <span class="hljs-string">f'https://finance.yahoo.com/quote/<span class="hljs-subst">{symbol}</span>/history/'</span>
    driver.get(url + <span class="hljs-string">'?period1=1579651200&amp;period2=9999999999'</span>)
    <span class="hljs-keyword">if</span> buttons := driver.find_elements(<span class="hljs-string">'xpath'</span>, <span class="hljs-string">'//button[@name="reject"]'</span>):
        buttons[<span class="hljs-number">0</span>].click()
    html = io.StringIO(driver.page_source)
    dataframes = pd.read_html(html, parse_dates=[<span class="hljs-string">'Date'</span>])
    s = dataframes[<span class="hljs-number">0</span>].set_index(<span class="hljs-string">'Date'</span>).Open
    <span class="hljs-keyword">return</span> s.rename(name)

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">wrangle_data</span><span class="hljs-params">(covid, bitcoin, gold, dow)</span>:</span>
    df = pd.concat([bitcoin, gold, dow], axis=<span class="hljs-number">1</span>)  <span class="hljs-comment"># Creates table by joining columns on dates.</span>
    df = df.sort_index().interpolate()            <span class="hljs-comment"># Sorts rows by date and interpolates NaN-s.</span>
    df = df.loc[<span class="hljs-string">'2020-02-23'</span>:<span class="hljs-string">'2021-12-20'</span>]        <span class="hljs-comment"># Keeps rows between specified dates.</span>
    df = (df / df.iloc[<span class="hljs-number">0</span>]) * <span class="hljs-number">100</span>                  <span class="hljs-comment"># Calculates percentages relative to day 1.</span>
    df = df.join(covid)                           <span class="hljs-comment"># Adds column with covid cases.</span>
    <span class="hljs-keyword">return</span> df.sort_values(df.index[<span class="hljs-number">-1</span>], axis=<span class="hljs-number">1</span>)   <span class="hljs-comment"># Sorts columns by last day's value.</span>

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">display_data</span><span class="hljs-params">(df)</span>:</span>
    figure = go.Figure()
    <span class="hljs-keyword">for</span> col_name <span class="hljs-keyword">in</span> reversed(df.columns):
        yaxis = <span class="hljs-string">'y1'</span> <span class="hljs-keyword">if</span> col_name == <span class="hljs-string">'Total Cases'</span> <span class="hljs-keyword">else</span> <span class="hljs-string">'y2'</span>
        trace = go.Scatter(x=df.index, y=df[col_name], yaxis=yaxis, name=col_name)
        figure.add_trace(trace)
    figure.update_layout(
        width=<span class="hljs-number">944</span>,
        height=<span class="hljs-number">423</span>,
        yaxis1=dict(title=<span class="hljs-string">'Total Cases'</span>, rangemode=<span class="hljs-string">'tozero'</span>),
        yaxis2=dict(title=<span class="hljs-string">'%'</span>, rangemode=<span class="hljs-string">'tozero'</span>, overlaying=<span class="hljs-string">'y'</span>, side=<span class="hljs-string">'right'</span>),
        colorway=[<span class="hljs-string">'#EF553B'</span>, <span class="hljs-string">'#636EFA'</span>, <span class="hljs-string">'#00CC96'</span>, <span class="hljs-string">'#FFA152'</span>],
        legend=dict(x=<span class="hljs-number">1.08</span>)
    )
    figure.show()

<span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:
    main()
</code></pre></div>



<div><h2 id="appendix"><a href="#appendix" name="appendix">#</a>Appendix</h2><div><h3 id="cython">Cython</h3><p><strong>Library that compiles Python-like code into C.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment"># $ pip3 install cython</span>
<span class="hljs-keyword">import</span> pyximport; pyximport.install()                <span class="hljs-comment"># Module that runs Cython scripts.</span>
<span class="hljs-keyword">import</span> &lt;cython_script&gt;                               <span class="hljs-comment"># Script must have '.pyx' extension.</span>
</code></pre></div></div>



<div><h4 id="allcdefdefinitionsareoptionalbuttheycontributetothespeedup">All <code class="python hljs"><span class="hljs-string">'cdef'</span></code> definitions are optional, but they contribute to the speed-up:</h4><pre><code class="python language-python hljs"><span class="hljs-keyword">cdef</span> &lt;type&gt; &lt;var_name&gt; [= &lt;obj/var&gt;]                 <span class="hljs-comment"># Either Python or C type variable.</span>
<span class="hljs-keyword">cdef</span> &lt;ctype&gt; *&lt;pointer_name&gt; [= &amp;&lt;var&gt;]              <span class="hljs-comment"># Use &lt;pointer&gt;[0] to get the value.</span>
<span class="hljs-keyword">cdef</span> &lt;ctype&gt;[size] &lt;array_name&gt; [= &lt;coll/array&gt;]     <span class="hljs-comment"># Also `&lt;ctype&gt;[:] &lt;mview&gt; = &lt;array&gt;`.</span>
<span class="hljs-keyword">cdef</span> &lt;ctype&gt; *&lt;array_name&gt; [= &lt;coll/array/pointer&gt;]  <span class="hljs-comment"># E.g. `&lt;&lt;ctype&gt; *&gt; malloc(n_bytes)`.</span>
</code></pre></div>

<pre><code class="python language-python hljs"><span class="hljs-keyword">cdef</span> &lt;type&gt; &lt;func_name&gt;(&lt;type&gt; [*]&lt;arg_name&gt;): ...   <span class="hljs-comment"># Omitted types default to `object`.</span>
</code></pre>
<pre><code class="python language-python hljs"><span class="hljs-keyword">cdef</span> <span class="hljs-class"><span class="hljs-keyword">class</span> &lt;<span class="hljs-title">class_name</span>&gt;:</span>                             <span class="hljs-comment"># Also `cdef struct &lt;struct_name&gt;:`.</span>
    <span class="hljs-keyword">cdef</span> <span class="hljs-keyword">public</span> &lt;type&gt; [*]&lt;attr_name&gt;                <span class="hljs-comment"># Also `... &lt;ctype&gt; [*]&lt;field_name&gt;`.</span>
    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, &lt;type&gt; &lt;arg_name&gt;)</span>:</span>           <span class="hljs-comment"># Also `cdef __dealloc__(self):`.</span>
        self.&lt;attr_name&gt; = &lt;arg_name&gt;                <span class="hljs-comment"># Also `... free(&lt;array/pointer&gt;)`.</span>
</code></pre>
<div><h3 id="virtualenvironments">Virtual Environments</h3><p><strong>System for installing libraries directly into project's directory.</strong></p><pre><code class="python hljs">$ python3 -m venv NAME         <span class="hljs-comment"># Creates virtual environment in current directory.</span>
$ source NAME/bin/activate     <span class="hljs-comment"># Activates it. On Windows run `NAME\Scripts\activate`.</span>
$ pip3 install LIBRARY         <span class="hljs-comment"># Installs the library into active environment.</span>
$ python3 FILE                 <span class="hljs-comment"># Runs the script in active environment. Also `./FILE`.</span>
$ deactivate                   <span class="hljs-comment"># Deactivates the active virtual environment.</span>
</code></pre></div>


<div><h3 id="basicscripttemplate">Basic Script Template</h3><p><strong>Run the script with <code class="python hljs"><span class="hljs-string">'$ python3 FILE'</span></code> or <code class="python hljs"><span class="hljs-string">'$ chmod u+x FILE; ./FILE'</span></code>. To automatically start the debugger when uncaught exception occurs run <code class="python hljs"><span class="hljs-string">'$ python3 -m pdb -cc FILE'</span></code>.</strong></p><pre><code class="python language-python hljs"><span class="hljs-comment">#!/usr/bin/env python3</span>
<span class="hljs-comment">#</span>
<span class="hljs-comment"># Usage: .py</span>
<span class="hljs-comment">#</span>

<span class="hljs-keyword">from</span> sys <span class="hljs-keyword">import</span> argv, exit
<span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> defaultdict, namedtuple
<span class="hljs-keyword">from</span> dataclasses <span class="hljs-keyword">import</span> make_dataclass
<span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum
<span class="hljs-keyword">import</span> functools <span class="hljs-keyword">as</span> ft, itertools <span class="hljs-keyword">as</span> it, operator <span class="hljs-keyword">as</span> op, re


<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">main</span><span class="hljs-params">()</span>:</span>
    <span class="hljs-keyword">pass</span>


<span class="hljs-comment">###</span>
<span class="hljs-comment">##  UTIL</span>
<span class="hljs-comment">#</span>

<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">read_file</span><span class="hljs-params">(filename)</span>:</span>
    <span class="hljs-keyword">with</span> open(filename, encoding=<span class="hljs-string">'utf-8'</span>) <span class="hljs-keyword">as</span> file:
        <span class="hljs-keyword">return</span> file.readlines()


<span class="hljs-keyword">if</span> __name__ == <span class="hljs-string">'__main__'</span>:
    main()
</code></pre></div>


<div><h2 id="index"><a href="#index" name="index">#</a>Index</h2><ul><li><strong>Ctrl+F / ⌘F is usually sufficient.</strong></li>
<li><strong>Searching <code class="python hljs"><span class="hljs-string">'#&lt;title&gt;'</span></code> will limit the search to the titles.</strong></li>
<li><strong>Click on the title's <code class="python hljs"><span class="hljs-string">'#'</span></code> to get a link to its section.</strong></li>
</ul></div>
 

  <footer>
    <aside>November 20, 2025</aside>
    <a href="https://gto76.github.io" rel="author">Jure Šorn</a>
  </footer>

  <a href="javascript:" id="return-to-top"><i class="icon-chevron-up"></i></a>

  <script src="web/jquery-3.4.0.min.js"></script>
  <script src="web/script_2.js"></script>
  <script type="text/javascript" src="https://transactions.sendowl.com/assets/sendowl.js" ></script>
  <script src="web/plotly.min.js"></script>
  <script src="web/covid_deaths.js"></script>
  <script src="web/covid_cases.js"></script>
</body>

</html>
